Add customization documentation and wp_djot_converter filter

- Add docs/customization.md with examples from djot-php cookbook
- Add wp_djot_converter filter for custom render handlers
- Add wp_djot_converter_{post_type} filter for per-post-type customization
- Examples: @mentions, #hashtags, lazy images, admonitions, heading anchors

Author: dereuromark

README.md

@@ -91,6 +91,7 @@ composer install --no-dev
 
 - [Usage & Configuration](docs/README.md) - Getting started guide
 - [Content Profiles](docs/profiles.md) - Configure feature restrictions
+- [Customization](docs/customization.md) - Custom patterns, @mentions, render handlers
 - [WP-CLI Commands](docs/wp-cli.md) - Migrate existing content
 - [Hooks and Filters](docs/hooks.md) - Customize plugin behavior
 - [Djot Syntax](docs/syntax.md) - Quick reference

docs/README.md

@@ -89,6 +89,7 @@ When installed, HTMLPurifier will automatically be used for sanitizing comment o
 ## More Documentation
 
 - [Content Profiles](profiles.md) - Configure feature restrictions for posts and comments
+- [Customization](customization.md) - Extend Djot with custom patterns, render handlers, @mentions, and more
 - [WP-CLI Commands](wp-cli.md) - Migrate existing content to Djot
 - [Hooks and Filters](hooks.md) - Customize plugin behavior
 - [Djot Syntax](syntax.md) - Quick reference for Djot markup