docs: promote "Build for Mobile" to top-level section + new pages

[rodrigo685]

Summary

Promotes Build for Mobile to a top-level navigation section, restructures it into three sub-sections (Mobile Client / Develop / Publish), and adds several new reference pages. Also removes outdated copy and TestFlight beta references.

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1. Promote "Build for Mobile" to top-level section (creator/SUMMARY.md)

• Removed "Building for Mobile" from being nested under Scenes (SDK7)
• Promoted it to its own ## Build for Mobile top-level section
• Restructured into three sub-sections: Mobile Client, Develop, Publish

2. Remove outdated tip from Onscreen UI (creator/sdk7/2d-ui/onscreen-ui.md)

• Removed the sentence: "A useful starting point is to design your UI on desktop and then scale sizes by 3× for mobile readability."

3. Add Hardware Requirements page (creator/sdk7/building-for-mobile/hardware-requirements.md)

• New page under Mobile Client
• Android and iOS minimum/recommended specs tables

4. Remove TestFlight hint block from Build for Mobile overview (creator/sdk7/building-for-mobile/README.md)

• Removed the beta tester TestFlight callout block
• Added mobile app screenshot

5. Remove TestFlight hint block from Player mobile app page (player/mobile-app/README.md)

• Removed the beta testing TestFlight callout block
• Added mobile app screenshot

6. Add Missing Features page (creator/sdk7/building-for-mobile/missing-features.md)

• New page under Mobile Client tracking the feature gap between the desktop (Unity) client and the mobile app
• Sourced from the godot-explorer feature parity tracker
• Four sections: SDK Features Missing on Mobile, Desktop Client Features Not in Mobile, Cross-Platform Inconsistencies, Input/Platform Constraints
• All issue references expanded to full URLs with real issue titles

7. Add Sample Scenes page (creator/sdk7/building-for-mobile/sample-scenes.md)

• New page under Mobile Client listing open-source mobile-optimized scenes from dcl-regenesislabs:
  • Dead Surge, Cozy Farm, Kick Off, Raft Game, Venetian Hunt, Tower of Madness

8. Add Optimize Performance page (creator/sdk7/building-for-mobile/optimize-performance.md)

• New page under Develop covering mobile scene limits and performance targets
• Scene limits table (12 metrics)
• Preview Mobile Performance section: how to use the Scene Limits Preview in Creator Hub
• Info hint explaining the Performance metric (% of FPS budget vs. graphic profile limit)
• Warning hint: keep Performance above 80% across all Graphic Profiles on a Samsung Galaxy A54
• Info hint: Switching Graphic Profiles path (Menu → Settings → Graphics)

9. Add Mobile Client section with Overview page (creator/sdk7/building-for-mobile/mobile-client/)

• New section index (Mobile Client) with landing page
• Overview page restoring the full content from the original building-for-mobile page
• Sections: Reference (Sample Scenes, Missing Features, Hardware Requirements), Develop, Publish
• Quick checklist for publishing mobile-ready scenes

10. Add Develop and Publish section indexes

• develop/README.md — index linking Detect Platform, Preview on Mobile, Safe Area, UI Best Practices, Input on Mobile, Performance
• publish/README.md — index linking Get Featured, iOS Curation

11. Add image to Get Featured page (creator/sdk7/building-for-mobile/get-featured.md)

• Added mobile-featured.png below the intro paragraph

12. Fix image paths

• Both uploaded images (mobile-app-screenshot.png, mobile_scene_limits.png, mobile-featured.png) live at the repo root — corrected all relative paths across affected files

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Files changed

File	Change
creator/SUMMARY.md	Restructured — Build for Mobile promoted to top-level with Mobile Client / Develop / Publish sub-sections
creator/sdk7/2d-ui/onscreen-ui.md	Removed outdated 3× scale tip
creator/sdk7/building-for-mobile/README.md	Removed TestFlight hint block + added screenshot
creator/sdk7/building-for-mobile/hardware-requirements.md	New — Android/iOS spec tables
creator/sdk7/building-for-mobile/missing-features.md	New — Desktop vs mobile feature gap tracker
creator/sdk7/building-for-mobile/sample-scenes.md	New — Open-source mobile scene references
creator/sdk7/building-for-mobile/optimize-performance.md	New — Mobile scene limits, performance preview, and targets
creator/sdk7/building-for-mobile/get-featured.md	Added mobile-featured.png image
creator/sdk7/building-for-mobile/mobile-client/README.md	New — Mobile Client section index
creator/sdk7/building-for-mobile/mobile-client/overview.md	New — Full overview with download links, section map, and quick checklist
creator/sdk7/building-for-mobile/develop/README.md	New — Develop section index
creator/sdk7/building-for-mobile/publish/README.md	New — Publish section index
player/mobile-app/README.md	Removed TestFlight hint block + added screenshot