docs: document Conditional extender, ApplicationBooted event, and LaravelServiceProvider support

[imorland]

Summary

Covers three undocumented additions from the beta.4–beta.7 cycle (identified in a gap review against the framework changelog).

Extend\Conditional — beta.4 (#4287)

• New ## Conditional Extenders section in settings.md
• Documents all four methods: whenExtensionEnabled, whenExtensionDisabled, whenSetting, and the underlying when
• Covers invokable class string form, loose vs strict comparison for whenSetting, and container-resolved callables for when
• Ref: [2.x] feat: conditional setting extender framework#4287

Flarum\Foundation\Event\ApplicationBooted — beta.7 (#4366)

• New ## Notable Core Events section in backend-events.md with a ### ApplicationBooted entry
• Explains timing (fires after all service providers and extenders, before any request), appropriate use cases, and includes a warning that it fires on every request
• service-provider.md boot process list updated with step 6 linking to this event
• Ref: [2.x] feat: fire ApplicationBooted after all callbacks have completed framework#4366

ServiceProvider extender accepts Illuminate\Support\ServiceProvider — beta.6 (#4349)

• service-provider.md updated to note that bare Laravel service providers are now accepted
• Added example showing a third-party Laravel package provider being registered directly
• Ref: [2.x] chore: allow LaravelServiceProvider or AbstractServiceProvider framework#4349

Test plan

• Verify Conditional::whenSetting loose/strict comparison examples against [2.x] feat: conditional setting extender framework#4287
• Verify ApplicationBooted timing description against [2.x] feat: fire ApplicationBooted after all callbacks have completed framework#4366
• Verify ServiceProvider extender accepts Illuminate\Support\ServiceProvider against [2.x] chore: allow LaravelServiceProvider or AbstractServiceProvider framework#4349

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