doc: make manpage URLs configurable based on release type

Add configurable documentation URLs that change based on whether this is
an official release or development build:

- Nix builds:
  - Development (officialRelease = false): Use /latest/ URLs
  - Official releases (officialRelease = true): Use versioned URLs with
    MAJOR.MINOR only (e.g., /2.33/ instead of /2.33.0/)
- Plain meson builds: Default to versioned URLs (official-release = true)

Changes:
- Add --doc-url parameter to expand-includes.py
- Add meson option 'official-release' (defaults to true for Meson builds)
- Compute doc_url in meson.build based on version and official-release
- Forward Nix officialRelease variable to Meson in package.nix
- Update render-manpage.sh to pass doc-url parameter

This allows distros (Fedora, etc.) to have stable versioned URLs by default,
while Nix development builds point to /latest/ for up-to-date documentation.

doc/manual/render-manpage.sh

@@ -15,20 +15,22 @@ if [ "$1" = --out-no-smarty ]; then
     shift
 fi
 
-[ "$#" = 6 ] || {
+[ "$#" = 7 ] || {
     cat >&2 <<EOF
-Usage: $0 [--out-no-smarty] <title> <section> <source-root> <generated-root> <infile> <outfile>
+Usage: $0 [--out-no-smarty] <title> <section> <source-root> <generated-root> <doc-url> <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
+  doc-url         - Base URL for documentation links
   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 \\
+     https://nix.dev/manual/nix/latest \\
      build/doc/manual/source/command-ref/nix-store/query.md nix-store-query.1
 EOF
     exit 1
@@ -38,8 +40,9 @@ title="$1"
 section="$2"
 source_root="$3"
 generated_root="$4"
-infile="$5"
-outfile="$6"
+doc_url="$5"
+infile="$6"
+outfile="$7"
 
 # Expand includes and pipe to lowdown
 (
@@ -47,5 +50,6 @@ outfile="$6"
     python3 "$script_dir/expand-includes.py" \
         --source-root "$source_root" \
         --generated-root "$generated_root" \
+        --doc-url "$doc_url" \
         "$infile"
 ) | lowdown -sT man --nroff-nolinks $lowdown_args -M section="$section" -o "$outfile"