Add an admin-guide.html page that lists all admin guides for all
projects that have them.
Replace the link to the central admin guide on the main page with a link
to the new templated page.
Add a flag to the project data schema to indicate when a project has an
admin guide.
Update the project data to set the has_admin_guide flag for projects
that already have them published.
Change-Id: I514fe5034c26a42cfb4225d521691c32c0ba9ef4
Signed-off-by: Doug Hellmann <doug@doughellmann.com>