Move static docs into master branch

The hand-maintained static HTML docs for DevStack have been in a
GitHub gh-pages branch; move them into the master branch in
preparation for hosting them in openstack.org infrastructure.

By default tools/build_docs.sh now builds the static HTML output
into docs/html.

Change-Id: Ide5f7d980a294c7a9f8a3decaed0939f1c239934
Merge "Move static docs into master branch"