docs: add Android auto-update page (Maui & Avalonia) (#69)

- Document both GeneralUpdate.Maui.Android and GeneralUpdate.Avalonia.Android
- Cover full update lifecycle: validate -> download -> verify -> install
- Include runnable code examples for both frameworks
- Provide AndroidManifest / FileProvider XML configuration
- Add Maui vs Avalonia comparison table with selection guidance
- Document server requirements and FAQ section
- Sync base docs, zh-Hans i18n, and en i18n copies

Closes #68

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: JusterZhu