Add markdown serving for AI-readable documentation

[Claude]

Implements clean markdown endpoints for all documentation pages to enable better AI tool parsing. Each page becomes available at URL.md (e.g., /getting-started/clients.md) with navigation, footers, and theme elements stripped.

Changes

• Jekyll plugin (_plugins/markdown_generator.rb) - Automatically generates .md versions during build by creating shadow pages with clean markdown layout
• Clean layout (_layouts/markdown.html) - Renders page title, description, and content only, no chrome
• GitHub Actions workflow (.github/workflows/pages.yml) - Custom build pipeline supporting Jekyll plugins (GitHub Pages default build doesn't support custom plugins)
• Theme configuration - Switched from remote_theme to local theme in _config.yml to enable plugin usage
• Dependencies (Gemfile) - Added just-the-docs and required Jekyll plugins

Limitations

Content negotiation via Accept: text/markdown header requires server-side routing unavailable in GitHub Pages static hosting. The .md URL suffix provides equivalent functionality without server configuration.

Example

HTML: https://docs.falkordb.com/cypher/indexing/vector-index
Markdown: https://docs.falkordb.com/cypher/indexing/vector-index.md

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Documentation AI Readable - Serve .md files</issue_title>
<issue_description>- [ ] Serve markdown versions of your doc pages
For each documentation page, serve a .md version at the same URL with .md appended. AI tools can parse markdown much better than HTML.

• Fix your markdown page responses
  Your .md URLs should return valid markdown content, not HTML or error pages. Check that your server properly handles .md file requests.

• Support Accept: text/markdown content negotiation
  When a request includes an Accept: text/markdown header, your doc pages should return markdown content instead of HTML. This lets AI tools request the format they prefer without needing a separate .md URL.

• Clean up your markdown output
  Your markdown versions include navigation, footer, or other non-content elements. Strip these to give AI clean, focused content.</issue_description>

Comments on the Issue (you are @claude[agent] in this section)

• Fixes Documentation AI Readable - Serve .md files #362