Moved the apidocs from openstack/database-api

By hosting the docs here we'll be able to better maintain that
developers are being mindful of changes or even occassionally writing
documentation for new API features. This should also allow us to
autogenerate the snippets.

Change-Id: I0111bcd38341b1736a47b0a3d3e6ff96d2ca1c58
(cherry picked from commit 294330c554eff8e2b120d2e4a2d39a44412de200)