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: I4fbe7838b6bada570f5b321d41deeb9aae9a0a73
Merge branch 'cinder-backends' into dt-test
Merge branch 'move-docs-into-repo' into dt-test
Merge branch 'usb-prereq' into dt-test
Merge branch 'conf-tempest' into dt-test