Switch to sphinx-test-reports for generating a test coverage report for the docs

[jackleland]

The link to the html coverage report generated as part of the test gh-action does not get properly served by gh-pages, a different approach will be needed.

An alternative approach is probably needed, likely something like this which builds from an xml file which can be output from pytest. In order to get this working then as part of the ci, the following steps will need to be followed:

1. Change the coverage report to be output in xml format
2. Install the sphinx extension correctly
3. Insert the appropriate command into the relevant bit of the docs
4. Check this works locally and update the README if it changes how to build the docs locally.