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/render-manpage.sh

@@ -1,25 +1,51 @@
 #!/usr/bin/env bash
+#
+# Standalone manpage renderer that doesn't require mdbook.
+# Uses expand-includes.py to preprocess markdown, then lowdown to generate manpages.
 
 set -euo pipefail
 
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
 lowdown_args=
 
+# Optional --out-no-smarty flag for compatibility with nix_nested_manpages
 if [ "$1" = --out-no-smarty ]; then
     lowdown_args=--out-no-smarty
     shift
 fi
 
-[ "$#" = 4 ] || {
-    echo "wrong number of args passed" >&2
+[ "$#" = 6 ] || {
+    cat >&2 <<EOF
+Usage: $0 [--out-no-smarty] <title> <section> <source-root> <generated-root> <infile> <outfile>
+
+Arguments:
+  title           - Manpage title (e.g., "nix-env --install")
+  section         - Manpage section number (1, 5, 8, etc.)
+  source-root     - Root directory of markdown sources
+  generated-root  - Root directory of generated markdown files
+  infile          - Input markdown file (relative to build directory)
+  outfile         - Output manpage file
+
+Examples:
+  $0 "nix-store --query" 1 doc/manual/source build/doc/manual/source \\
+     build/doc/manual/source/command-ref/nix-store/query.md nix-store-query.1
+EOF
     exit 1
 }
 
 title="$1"
 section="$2"
-infile="$3"
-outfile="$4"
+source_root="$3"
+generated_root="$4"
+infile="$5"
+outfile="$6"
 
+# Expand includes and pipe to lowdown
 (
     printf "Title: %s\n\n" "$title"
-    cat "$infile"
+    python3 "$script_dir/expand-includes.py" \
+        --source-root "$source_root" \
+        --generated-root "$generated_root" \
+        "$infile"
 ) | lowdown -sT man --nroff-nolinks $lowdown_args -M section="$section" -o "$outfile"