Use Github Actions to auto-generate user guide HTML

[ferdnyc]

We're running the docs build at the end of each Travis run right now, which is fine for CI purposes, but not very convenient.

Using Github Actions, instead, lets the results of the run be exported as an Artifact from the job, so that the generated HTML tree can be downloaded as a ZIP file right from the Actions status page. Handy for documentation contributors, so they don't have to go through the hassle of setting up to run Sphinx on their own PCs.

A tweak to src/classes/info.py makes it handle PyQt5 being unavailable (as it will be, in the Actions environment), and the requirements.txt file added to doc/ is a standard config defining the Python module dependencies for our Sphinx config. The Actions task will ensure that all dependencies are met before starting the build.

[ferdnyc]

We're running the docs build at the end of each Travis run right now,

Actually, I take it back, I guess we're not doing that! Well, even better then.

[ferdnyc]

Merging as this is primarily new configs, and the classes.info change just wraps a try: block around part of the code.