Add example code for doxygen groups
Add doxygengroup directive

We copy the doxygenfile directive for the moment. They should probably
be refactored to share implementations later.

We introduce a new filter and update the filter code to allow for the
selection of a node in the index.xml file rather than targeting the
compound.xml. The filter looks for the top level compound node of
kind=group which has the name we're interested in.

Having found this we then render it and everything under it.

Improvements:
- We should provide an option to allow the group itself to be ignored
  in case people just want to use groups as a mechanism for grouping
  content but not have them represented in the final docs.
Add doxygengroup directive to the docs

With example.
Add local table of contents to directives page

To make it more easily navigable. And remove redundancy in the titles.
Add :content-only: functionality to doxygengroup

This seems to work but is distracted commit done to backup the work
before my computer is shipped around the world. I'm not in the best
place to write a good commit message.

:content-only: on a doxygengroup directive should mean that it only
renders the content of the group and doesn't include the group name and
group-specific documentation.
Merge branch 'doxygengroup'