Clean up Kotlin and annotation guidelines
Clean up Kotlin and annotation guidelines

- Add guidance on extending platform APIs
- Link to top-level async guidelines

PiperOrigin-RevId: 470751908
Change-Id: I149b8022ffff5d48925bcc7175d6c7f6ea5c27f0

androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx