docs: add deprecated branch banner

The master branch is dead.  Add banners to all the docs in case people
try referring to these and don't realize they're on the wrong branch.

Change-Id: I3488d0d96df25fafd7285848fe9f519b4205519c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/400918
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>