various improvements to docs

[2bndy5]

This includes spelling fixes, updates for the latest release of sphinx-immaterial theme, and fixes #841 (as per feedback provided there).

I also reduced the Doxyfile to only options that we change from their default value. This should help avoid warnings in future Doxygen output.

Note to devs: We cannot use Doxygen v1.9.3 (& possibly v1.9.2) due to erroneous XML output that broke compatibility with Sphinx's breathe extension. Recommend v1.9.4+

In reference to #814 The sphinx docs use the prototype idea from that thread (see the RF24 API page). Doxygen is proving very inflexible in solving that problem (so far).

[TMRh20]

Looks good.
One thing thats missing from the CMAKE instructions is the installer you created. Do you want to add that to the CMAKE docs? I think some users may prefer to use it to install all the libraries.

[2bndy5]

The automated install instruction still exist in the old Linux install doc. It would be appropriate to move them to the CMake doc since it is now based on CMake. I'd probably leave a link to the automated install instructions in its place on the old Linux install doc.

Or I could add a link to the CMake doc to the automated install instructions... But that would be the lazy approach.

[TMRh20]

The automated install instruction still exist in the old Linux install doc.

I don't see a link to the new installer, just the old installer using makefiles etc.. and instructions for that. I'd like to keep both installers now that we have them - there are older tutorials etc linking to the old installer as well.

[2bndy5]

Oops, I thought you changed the link to the auto-installer. If you want to keep both, then that makes the request easier.