Improve GitHub and Doxygen docs.

[dbaarda]

Improve GitHub and Doxygen docs.

[dbaarda]

Other things I've noticed;

1. Currently .gitattributes marks *.in files as binary so that test data using this extension doesn't get modified on Windows checkouts and break tests. Unfortunately this extension is also used for some cmake template files, including doc/Doxyfile.in. Interestingly src/config.h.cmake doesn't use .in but it probably should since .cmake indicates a cmake config file, not a template. We should rename the test data to use .input instead, and rename src/config.h.cmake to src/config.h.in while we are at it.
2. In doc/Doxyfile.in there is a setting HAVE_DOT = NO. This turns of all the nice diagrams that Doxygen can generate. Changing this to on makes the docs much more useful. There is also DOT_IMAGE_FORMAT = png, and using svg would probably be better. Also maybe interesting would be INTERACTIVE_SVG = YES to add interactive zoom/pan. Note this requires gaphviz to be installed, so we should update the GitHub action to install it.
3. There are many files missing the basic \file <foo>.h Doxygen docstring which means they are omitted from the docs. Adding at least all the headers would make navigating the include diagrams much nicer.
4. Some functions are documented in the *.h file, and some in the *.c file. We should put documentation in the *.h file when we can.

[dbaarda]

After playing around with markdown docs and github actions in #230 I think maybe replacing \ref links with markdown links may be premature. It seems the version of Doxygen on the github actions ubuntu_latest target suffers from doxygen/doxygen#7595 so markdown links don't quite work as advertised.

There are still several things here that need fixing, so I'm going to put together a PR with the bits still worth doing.