doc: make manpage generation independent of mdbook

Add standalone markdown preprocessor to generate manpages without requiring
mdbook's Rust toolchain. This removes a significant build dependency for
manpage generation while keeping the HTML manual (mdbook) working unchanged.

Changes:
- Add expand-includes.py: Python 3 script that recursively expands
  {{#include}} directives, resolves @docroot@ to nix.dev URLs, and handles
  @generated@/ paths for build-generated files
- Update render-manpage.sh: Replace mdbook-based implementation with
  standalone version that uses expand-includes.py + lowdown
- Update meson.build: All 134 manpage targets now use standalone renderer
  with proper dependencies (expand-includes.py, experimental-features-shortlist)
- Fix nix-hash.md: Remove extra parenthesis in markdown link syntax

Benefits:
- No mdbook/Rust toolchain required for manpage builds
- Manpages contain nix.dev/latest URLs instead of broken relative paths
- Fixes bug where mdbook didn't expand experimental-features-shortlist.md
- 98.5% identical output to mdbook (2 files differ, both acceptable)

All 134 manpages (131 section 1, 2 section 5, 1 section 8) build successfully.

doc/manual/expand-includes.py

@@ -0,0 +1,220 @@
+#!/usr/bin/env python3
+"""
+Standalone markdown preprocessor for manpage generation.
+
+Expands {{#include}} directives and handles @docroot@ references
+without requiring mdbook.
+"""
+
+from pathlib import Path
+import sys
+import argparse
+import re
+
+
+def expand_includes(
+    content: str,
+    current_file: Path,
+    source_root: Path,
+    generated_root: Path | None,
+    visited: set[Path] | None = None,
+) -> str:
+    """
+    Recursively expand {{#include path}} directives.
+
+    Args:
+        content: Markdown content to process
+        current_file: Path to the current file (for resolving relative includes)
+        source_root: Root of the source directory
+        generated_root: Root of generated files (for @generated@/ includes)
+        visited: Set of already-visited files (for cycle detection)
+    """
+    if visited is None:
+        visited = set()
+
+    # Track current file to detect cycles
+    visited.add(current_file.resolve())
+
+    lines = []
+    include_pattern = re.compile(r'^\s*\{\{#include\s+(.+?)\}\}\s*$')
+
+    for line in content.splitlines(keepends=True):
+        match = include_pattern.match(line)
+        if not match:
+            lines.append(line)
+            continue
+
+        # Found an include directive
+        include_path_str = match.group(1).strip()
+
+        # Resolve the include path
+        if include_path_str.startswith("@generated@/"):
+            # Generated file
+            if generated_root is None:
+                raise ValueError(
+                    f"Cannot resolve @generated@ path '{include_path_str}' "
+                    f"without --generated-root"
+                )
+            include_path = generated_root / include_path_str[12:]
+        else:
+            # Relative to current file
+            include_path = (current_file.parent / include_path_str).resolve()
+
+        # Check for cycles
+        if include_path.resolve() in visited:
+            raise RuntimeError(
+                f"Include cycle detected: {include_path} is already being processed"
+            )
+
+        # Check that file exists
+        if not include_path.exists():
+            raise FileNotFoundError(
+                f"Include file not found: {include_path_str}\n"
+                f"  Resolved to: {include_path}\n"
+                f"  From: {current_file}"
+            )
+
+        # Recursively expand the included file
+        included_content = include_path.read_text()
+        expanded = expand_includes(
+            included_content,
+            include_path,
+            source_root,
+            generated_root,
+            visited.copy(),  # Copy visited set for this branch
+        )
+        lines.append(expanded)
+        # Add newline if the included content doesn't end with one
+        if not expanded.endswith('\n'):
+            lines.append('\n')
+
+    return ''.join(lines)
+
+
+def resolve_docroot(content: str, current_file: Path, source_root: Path) -> str:
+    """
+    Replace @docroot@ with nix.dev URL and convert .md to .html.
+
+    For manpages, absolute URLs are more useful than relative paths since
+    manpages are viewed standalone. lowdown will display these as proper
+    references in the manpage output.
+    """
+    # Use the latest nix.dev documentation URL
+    # This matches what users would actually want to reference from a manpage
+    docroot_url = "https://nix.dev/manual/nix/latest"
+
+    # Replace @docroot@ with the base URL
+    content = content.replace("@docroot@", docroot_url)
+
+    # Convert .md extensions to .html for web links
+    # Use lookahead to ensure that .md occurs before a fragment or a possible URL end.
+    content = re.sub(
+        r'(https://nix\.dev/[^)\s]*?)\.md(?=[#)\s]|$)',
+        r'\1.html',
+        content
+    )
+
+    return content
+
+
+def resolve_at_escapes(content: str) -> str:
+    """Replace @_at_ with @"""
+    return content.replace("@_at_", "@")
+
+
+def process_file(
+    input_file: Path,
+    source_root: Path,
+    generated_root: Path | None,
+) -> str:
+    """Process a single markdown file."""
+    content = input_file.read_text()
+
+    # Expand includes
+    content = expand_includes(content, input_file, source_root, generated_root)
+
+    # Resolve @docroot@ references
+    content = resolve_docroot(content, input_file, source_root)
+
+    # Resolve @_at_ escapes
+    content = resolve_at_escapes(content)
+
+    return content
+
+
+def main():
+    parser = argparse.ArgumentParser(
+        description="Expand markdown includes for manpage generation",
+        formatter_class=argparse.RawDescriptionHelpFormatter,
+        epilog="""
+Examples:
+  # Expand a manpage source file
+  %(prog)s \\
+    --source-root doc/manual/source \\
+    --generated-root build/doc/manual/source \\
+    doc/manual/source/command-ref/nix-store/query.md
+
+  # Pipe to lowdown for manpage generation
+  %(prog)s -s doc/manual/source -g build/doc/manual/source \\
+    doc/manual/source/command-ref/nix-env.md | \\
+    lowdown -sT man -M section=1 -o nix-env.1
+        """,
+    )
+    parser.add_argument(
+        "input_file",
+        type=Path,
+        help="Input markdown file to process",
+    )
+    parser.add_argument(
+        "-s", "--source-root",
+        type=Path,
+        required=True,
+        help="Root directory of markdown sources",
+    )
+    parser.add_argument(
+        "-g", "--generated-root",
+        type=Path,
+        help="Root directory of generated files (for @generated@/ includes)",
+    )
+    parser.add_argument(
+        "-o", "--output",
+        type=Path,
+        help="Output file (default: stdout)",
+    )
+
+    args = parser.parse_args()
+
+    # Validate paths
+    if not args.input_file.exists():
+        print(f"Error: Input file not found: {args.input_file}", file=sys.stderr)
+        return 1
+
+    if not args.source_root.is_dir():
+        print(f"Error: Source root is not a directory: {args.source_root}", file=sys.stderr)
+        return 1
+
+    if args.generated_root and not args.generated_root.is_dir():
+        print(f"Error: Generated root is not a directory: {args.generated_root}", file=sys.stderr)
+        return 1
+
+    try:
+        # Process the file
+        output = process_file(args.input_file, args.source_root, args.generated_root)
+
+        # Write output
+        if args.output:
+            args.output.write_text(output)
+        else:
+            print(output, end='')
+
+        return 0
+
+    except Exception as e:
+        print(f"Error processing {args.input_file}: {e}", file=sys.stderr)
+        import traceback
+        traceback.print_exc(file=sys.stderr)
+        return 1
+
+
+if __name__ == "__main__":
+    sys.exit(main())