[Docs] Restructure SideBar (#4365)

## Description

This PR:

- Extracts `GestureHandlerRootView` and `GestureDetector` out of
Fundamentals into a new "Core Components" section
- Moves State manager from Fundamentals to Gestures
- Renames "Quick start" to "Your first gesture-driven animation" and
moves it under Fundamentals
- Renames the Gestures category to "Gestures API"
- Fixes page ordering (sidebar_position) across Components/Guides
- Adds redirects for all removed/moved pages (gesture-detectors,
root-view, quickstart, state manager)
- Updates mapLegacyUrl.js so  /quickstart URLs resolve correctly
- Fixes ~30 internal doc links broken by the moves above

## Test plan

- `yarn build`
- Read docs 🤓