Simplify reserved regions docs

[NicooleT]

Summary

The documentation repeated setup and measurement details across pages and introduced provider geometry before explaining regions. This update gives readers a shorter getting-started guide, introduces divisions and occlusions before Usage, and uses consistent wording throughout.

• Streamline Getting Started and Usage, including the rendering gate example.
• Explain divisions, occlusions, region frames, and iOS/Android behavior with code examples and mapping tables.
• Shorten the safe area and example guides; consolidate the separate performance page into Usage and update links.
• Move “How regions work” before Usage and rename Coordinate spaces to Understanding regions.
• Update the navigation logo to logo.png and the primary color to #008CFF.