Add the doc build tools

tools/build_docs.sh generates the devstack.org website from the static
pages and generated pages created by running shocco against a DevStack
checkout.

Note that while this is the complete auto page generation of the
devstack.org site, pushing the content back to GitHub is limited
to those with push access to the current repo.

Partial-bug 1235626

Change-Id: I61dc3d56e4a4832a9ddd1904dd8af65c15a17e50
Merge "Add the doc build tools"