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/source/command-ref/nix-hash.md

@@ -34,7 +34,7 @@ md5sum`.
   Print the cryptographic hash of the contents of each regular file *path*.
   That is, instead of computing
   the hash of the [Nix Archive (NAR)](@docroot@/store/file-system-object/content-address.md#serial-nix-archive) of *path*,
-  just [directly hash]((@docroot@/store/file-system-object/content-address.md#serial-flat) *path* as is.
+  just [directly hash](@docroot@/store/file-system-object/content-address.md#serial-flat) *path* as is.
   This requires *path* to resolve to a regular file rather than directory.
   The result is identical to that produced by the GNU commands
   `md5sum` and `sha1sum`.