This one is a little unusual. The current theming doc mixes user-focused
documentation with developer-focused documentation. This is confusing
and unnecessary. Resolve the issue by splitting the doc in two. The
user-focused docs are moved to the advanced section of the usage guide,
while the developer docs remain where they are, pending the addition of
a developer guide.

Signed-off-by: Stephen Finucane <stephen@that.guru>