docs: Updating N Preview "Support and Release Notes" page

Page had been copied from the final M page (for M preview 3).
Stripped it down for the initial N release, and flagged areas where
we need more information (e.g. known issues? build number?).

See first comment for doc stage location.

bug: 27479193
Change-Id: I864ed7708ff37a453e195164e554e8cf8fc22687