This explicitly renames the cogl-2.0 reference manual to
cogl-2.0-experimental and renames the cogl-2.0 pkg-config file to
cogl-2.0-experimental.pc. Hopefully this should avoid
miss-understandings.
This pulls in the experimental cogl 2.0 reference manual from the
clutter repository since it wasn't included in the filter-branch when we
split cogl out.