feat: refresh documentation navigation and reading layout

[LunaStev]

Documentation now uses a reading layout inspired by Microsoft Learn: a collapsible, filterable navigation tree on the left, a wider article with clear heading hierarchy, and a section index that follows the current reading position. Focus mode centers the article. On narrow screens, navigation and the section index are available as expandable panels.

The documentation catalog shares the same navigation and uses grouped guide lists. Wave, Standard Library, and Whale remain separate selections, with the documentation language selector in the header.

Validation:

• Frontend production build and documentation tests.
• Docker Compose rebuild; local platform, gateway, and playground healthy.
• Browser checks for filtering collapsed groups, empty search results, keyboard expansion, section links, focus mode, and mobile navigation.
• Language changes, English fallback, all three project catalogs, and embedded Wave execution.
• Light/dark themes and 390, 768, 1024, 1440, and 1920px layouts without horizontal overflow.
• Whitespace checks and browser console checks.