Document iOS app extension targets

[Matildevoldsen]

Summary

• document the ios.extension_targets manifest shape introduced by Add WidgetKit extension target support for plugins mobile-air#174
• cover isolated sources, App Groups, WidgetKit timelines, environment allowlisting, and extension signing
• record the host entitlement merge semantics in the v4 changelog
• add a rendered-route regression test for the new page

Testing

• PHP syntax check passed for the updated docs route test
• Pint passed for the updated PHP test
• documentation content and relative-link checks passed
• the rendered-route test is included but could not be executed locally because the private Flux Pro Composer repository returned HTTP 401 during dependency installation

Dependency

This documentation targets the extension support in NativePHP/mobile-air#174 and should land with or after that PR.