Add workflow for generating C++ docs with Doxygen on GitHub Pages

[jessica-mitchell]

This PR creates a CI workflow that will deploy the doxygen C++ docs to GitHub Pages or create the output HTML as artifacts.

The workflow triggers in the following cases:

1. On push (merge to master): if there is a change to
   • any cpp file
   • the doxgyen config file (fulldoc.conf.in)
   • the doxygen css (doxgyen-aweseome.css)
   • the workflow file
2. On pull_request : if there is a change
   • to the same files above, except workflow
     In this case GitHub pages is not deployed, but the html output from doxygen is uploaded as an artifact

The developer documentation workflow description has been updated to explain this new feature. A screenshot might be added in a future PR to help find the artifact.

The workflow is set up so we always use the Doxygen config file and the css that is in master; this allows us to modify the type of content and the layout at any time and the changes will be deployed for all versions of the documentation.

Currently only master, and the latest 2 releases of NEST are deployed. We want to keep it minimal to discourage old docs discovery and utilization.

@terhorstd mentioned we might need to consider adding the pynestkernel.pyx which can be done at a future PR; only cpp/h files are processed at the moment.

[jessica-mitchell]

Note that the CI run fails because the files that it needs are not yet in master
Would it make sense to add these files in a separate PR to be merged prior to this one?