docs: Restoring file to allow staging reference docs from Gerrit

Previously deleted almost all the docs (with CL http://ag/1970807),
since those docs are now managed and staged/published from Piper.
However, reference docs are still generated in Gerrit, so it's useful
to be able to stage those docs from a Gerrit client--and that was
broken by my change, since the staging tool expects to see
_book.yaml and _project.yaml in the reference doc directory. So,
restoring those files to re-enable staging of reference docs from
Gerrit.

The publish process is *not* changed: we still grab a good build
from the appropriate Gerrit branch and migrate those files into
Piper, then publish the Piper files.

Test: make ds-docs ; devsite stage --site=android en/reference
Change-Id: I392f1699dc68767d1ac9b7113a149062e5e48ef7