Refresh platforms API status doc

This does a few things:

- Rename to make it clearer this is a migration / status review. vs. the stable documentation at https://bazel.build/extending/platforms.
- Updates status of language migrations
- Focus more on migration status, move deep-dive details to the end
- Various language simplifications and updates

**Old doc:** https://bazel.build/concepts/platforms

**New doc:** []

PiperOrigin-RevId: 523384501
Change-Id: I96bb53759f4c0b78679704647e1a1afb8150aa4a

Author: gregestren