Complete rewrite

### Renderer

This release contains a complete reimplementation of the renderer (!27). It now knows how to render:

- A navigation bar on each page
- Page metadata (available platforms)
- Headers
- Platform-specific content
- Code blocks
- Inline code
- Hyperlinks (local and external)
- Bold, italic and strikethrough text
- Package lists
- Type lists
- Function & constructor lists
- Property lists
- Inheritor lists
- Ordered and unordered lists
- The `@throws` and `@see` tags
- Type parameters

Known issues in this version:

- Hyperlinks in module header pages are broken. Links in all other pages should work.
- Platform-specific tabs are sometimes weird for expect/actual symbols.

### Documentation

- Added a mention that `use_directory_urls` is forbidden (#14, !29)
- Added a 'News' section which contains releases (!30)

### Dependencies

- Kotlin 2.1.10