This splits the Sphinx tag option from SPHINXOPTS. This allows us to
specify the tag without having to remember to add -W and -t. It also
fixes not being able to specify the tag with make.bat on Windows.
While we are touching this, redundant SOURCEDIR uses are removed (main
is already the default) and the .github/workflows/publish-ide-docs.yml
script is fixed (ide directory no longer exists).
With EV3 dropped from the 3.X documentation, the main docs are now nearly identical to the IDE docs, save for some layout changes.
Having just one set of docs is easier to maintain. This drops the IDE doc target.
MathJax uses a CDN to to distribute the package. So in order to use the
docs offline, we would have to include our own copy. But it is a bit
large (50-ish MB), so it is easier to use sphinx.ext.imgmath instead
to generate svg images.
The situation might be different with MathJax v3, but that requires
Sphinx v4.