This makes it feasible to merge the LV2 documentation into more broad
documentation in a meta-project.

This removes all the extra bundle content from the documentation output and
instead makes pages directly at the URI of specifications.  This way, links to
fragments in spec pages (which are often copy and pasted) are correct fragment
identifiers like <http://lv2plug.in/ns/lv2core#Plugin> instead of
<http://lv2plug.in/ns/lv2core/#Plugin>.

This order is better because it ensures that headers can stand alone and
include everything they need.  Several missing includes were exposed by this,
and are also fixed in this commit.