Update to Slate v2.13.1 from official slatedocs/slate fork

[cgoetze11]

Summary

Updates the API documentation infrastructure from the unmaintained 10-year-old ringcentral/slate fork to the official slatedocs/slate v2.13.1
release. This brings 150+ commits of improvements including updated dependencies, bug fixes, and better Middleman integration.

Key Changes

Infrastructure Updates

• Middleman: 4.2.1 → 4.4.3 (latest stable)
• Ruby: Updated to support Ruby 2.6-3.1 (tested with 3.1.6)
• Redcarpet: 3.4.0 → 3.6.0
• Rouge: Updated to 3.30.0
• Nokogiri: Updated to 1.13.9
• Added webrick: Required for Ruby 3.x compatibility

Guidebook Customizations Preserved

• Maintained deploy.sh --current VERSION flag for copying versioned index.html to root
• Kept custom unique header ID generation in lib/unique_head.rb
• Preserved existing markdown configuration and syntax highlighting

Compatibility Fixes

Redcarpet Renderer (Middleman 4.6)

Fixed undefined method 'link_to' for nil:NilClass error by switching from custom renderer class to monkey-patching approach in config.rb.
This ensures Middleman properly sets up the scope variable that the renderer needs.

Before (broken with Middleman 4.6):

set :markdown, renderer: UniqueHeadCounter

After (working):
ready do
  Middleman::Renderers::MiddlemanRedcarpetHTML.class_eval do
    def header(text, header_level)
      # Custom header ID logic
    end
  end
end

Build Optimizations

Temporarily disabled three build optimizations due to compatibility issues:

1. Autoprefixer: Disabled due to SCSS comments in compiled CSS
  - Sass :compressed output leaves // comments that autoprefixer can't parse
  - Future fix: Migrate to Dart Sass which properly strips all comments
2. CSS Minification: Disabled due to import resolution issues
  - Minifier attempts to recompile already-compiled CSS
  - Sass already outputs compressed CSS via style: :compressed
3. JS Minification: Disabled due to ES6 syntax
  - Uglifier doesn't support const, arrow functions, etc.
  - Future fix: Replace with terser for ES6+ support

Testing

- ✅ bundle install succeeds with Ruby 3.1.6
- ✅ bundle exec middleman build --clean completes successfully
- ✅ Generated HTML files (v1 and v1.1) are properly formatted
- ✅ All assets (CSS, JS, fonts, images) build correctly
- ✅ Custom header ID generation works as expected

Migration Notes

- Requires Ruby >= 2.6 (recommend 3.1.6 via rbenv)
- Run bundle install to update dependencies
- .ruby-version file added for rbenv users
- No changes to documentation source files required

Future Improvements

- Re-enable autoprefixer after Dart Sass migration
- Re-enable JS minification with terser
- Consider using slatedocs' nested header ID approach
- Evaluate middleman-syntax 3.3.0 upgrade (currently 3.2.0 for Haml compatibility)

Related Issues

Fixes compatibility with modern Ruby versions (2.7-3.1) and brings infrastructure up to date with actively maintained upstream.