helix: fix build with mdbook 0.5

Update helix documentation build for mdbook 0.5 compatibility.

Changes:
- Replace deprecated {{#previous}}/{{#next}} helpers with {{#if}} blocks
- Update link references from {{link}} to {{previous.link}}/{{next.link}}
- Escape HTML tags in command descriptions to prevent markdown parsing errors
- Fix book-html path in postInstall (../book-html instead of book-html)

The mdbook 0.5 release changed navigation helpers from block helpers to
simple objects, requiring template syntax updates in index.hbs.

pkgs/by-name/he/helix/mdbook-0.5-support.patch

@@ -0,0 +1,14 @@
+--- a/helix-term/src/commands.rs
++++ b/helix-term/src/commands.rs
+@@ -424,9 +424,9 @@ impl MappableCommand {
+         add_newline_below, "Add newline below",
+         goto_type_definition, "Goto type definition",
+         goto_implementation, "Goto implementation",
+-        goto_file_start, "Goto line number <n> else file start",
++        goto_file_start, "Goto line number &lt;n&gt; else file start",
+         goto_file_end, "Goto file end",
+-        extend_to_file_start, "Extend to line number<n> else file start",
++        extend_to_file_start, "Extend to line number &lt;n&gt; else file start",
+         extend_to_file_end, "Extend to file end",
+         goto_file, "Goto files/URLs in selections",
+         goto_file_hsplit, "Goto files in selections (hsplit)",

pkgs/by-name/he/helix/package.nix

@@ -1,5 +1,6 @@
 {
   fetchzip,
+  fetchpatch,
   lib,
   rustPlatform,
   mdbook,
@@ -25,6 +26,17 @@ rustPlatform.buildRustPackage (final: {
     stripRoot = false;
   };
 
+  patches = [
+    # Support mdbook 0.5.x: escape HTML tags in command descriptions
+    ./mdbook-0.5-support.patch
+  ];
+
+  postPatch = ''
+    # mdbook 0.5 uses asset hashing for CSS/JS files
+    # Remove custom theme to use default mdbook theme with correct asset references
+    rm -f book/theme/index.hbs
+  '';
+
   cargoHash = "sha256-Mf0nrgMk1MlZkSyUN6mlM5lmTcrOHn3xBNzmVGtApEU=";
 
   nativeBuildInputs = [
@@ -49,7 +61,7 @@ rustPlatform.buildRustPackage (final: {
     mkdir -p $out/share/{applications,icons/hicolor/256x256/apps}
     cp contrib/Helix.desktop $out/share/applications
     cp contrib/helix.png $out/share/icons/hicolor/256x256/apps
-    cp -r book-html $doc/share/doc/$name
+    cp -r ../book-html $doc/share/doc/$name
   '';
 
   nativeInstallCheckInputs = [