fix(scripts): improve nix options doc generator

- Restore missing description on hm enable option (fixes nix build warning)
- Rewrite format_default_value into format_value with example support and nix code blocks
- Fix Nix markup stripping: {tag}`content` now preserves backtick content
- Generalize ::: block handling beyond {.note} to any block type
- Switch output from Markdown table to labeled fields format

nix/hm-modules.nix

@@ -22,6 +22,7 @@ in
       enable = mkOption {
         type = types.bool;
         default = false;
+        description = "Whether to enable mangowm, a Wayland compositor based on dwl.";
       };
       package = lib.mkOption {
         type = lib.types.package;