docs(realtime): fix ext: import paths, clarify model selection, improve examples

[imorland]

Follow-up to #516.

Summary

• Fix all cross-extension import examples to use ext: prefix (e.g. ext:flarum/realtime/forum/extenders/Realtime) instead of the incorrect require('flarum/realtime/forum').RealtimeExtend form
• Fix RealtimeState import path to also use ext: prefix
• Add :::info callout explaining the ext: import convention and why it's required
• Add "Choosing which model to return" guidance explaining when to return $event->post vs a custom model
• Restructure the JS extender section to show the extendRealtime.ts file pattern clearly
• Fix the complete likes example to use correct import syntax throughout

These were committed to the previous branch after the PR was merged and the branch deleted.