Add docs for:
- cppgc namespace (to have doxygen generate the namespace doc)
- Heap
- LivenessBroker

Bug: chromium:1056170
Change-Id: I5e4664458b7209f4adebb4d5e7b5119c341f59a9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2214834
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68015}