[Ecosystem > TMA] Add theming, viewport, and UI components pages

This should be added:

- New "Theming" page: how to retrieve and apply Telegram theme colors, color reference
- New "Viewport" page: viewport properties, fullscreen mode, closing and swipe behavior
- New "UI components" page: native controls provided by Telegram (Back Button, Main Button, Settings Button, Popup)

Follow-up: Next PRs will cover dev workflow and SDK overview