Provide reference documentation

[claremacrae]

This is to implement the "Reference" quadrant of Daniele Procida's Documentation System:

https://documentation.divio.com

Sy Brand has documented a series of steps to implement the technical aspects of this for C++ code, to publish library documentation on Read the Docs:

https://devblogs.microsoft.com/cppblog/clear-functional-c-documentation-with-sphinx-breathe-doxygen-cmake/

[claremacrae]

There is a starting point available now at:

• https://approvaltestscpp.readthedocs.io/en/latest/

This is updated every time changes are pushed to GitHub.

The next step is to work out what to show to users, and then to set up a meaningful structure to show it.

[claremacrae]

TODOs:

• [x] Move the Doxygen and sphinx files to sub-directories, as the /doc/ directory is just too cluttered now
• [x] Work out what to show to users
• [x] Set up a meaningful structure for content in Sphinx

Note to self: Remember not to change files that have been changed significantly on the Options branch.