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>