feat(docs): publish themed API Reference GitHub Pages subpage

Turn the raw docs/api-reference.md (previously served as plain text by the
GitHub Pages Jekyll build) into a styled HTML subpage that matches the
project landing page.

- Add docs/api-reference.html: reuses the site header/footer and design
  tokens, adds a breadcrumb, hero, and a two-column layout with a sticky
  "On this page" table of contents.
- Add docs/doc-page.css: documentation layout and article typography
  (tables, callouts, inline code, dark code cards).
- Add docs/doc-page.js: scrollspy TOC highlighting, copy-to-clipboard
  buttons, collapsible mobile TOC, and highlight.js init with graceful
  degradation.
- Point the index.html Documentation card and footer link at the rendered
  api-reference.html instead of the raw Markdown file.

Content is generated from api-reference.md, so code samples and tables map
1:1 to the source; cross-document .md references now link to the rendered
GitHub blob view.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Kilo

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **Themed API Reference documentation page** — the developer API reference is now published as a styled GitHub Pages subpage (`docs/api-reference.html`) that matches the project landing page, with a sticky "On this page" table of contents, syntax-highlighted PHP samples, copy-to-clipboard buttons on every code block, and a responsive layout. The Documentation links now open the rendered page instead of the raw Markdown file.
 
 ### Changed