[python-qemu-qmp MR] Add Sphinx documentation

[John Snow]

I'm continuing to mail these manually for now, until I settle on an
automated solution or feel like there's enough buy-in on the GitLab
instance to not warrant the advertisement. Please consider
starring/watching the repo if you're interested in QMP lib updates as
I try to angle for an initial public release.

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MR: https://gitlab.com/qemu-project/python-qemu-qmp/-/merge_requests/4
From: https://gitlab.com/jsnow

Howdy! This MR:

- Adds sphinx documentation as a build artifact
- Ensures documentation builds without errors in the CI pipeline
(preventing stale cross-reference bitrot)
- Publishes docs to e.g. https://jsnow.gitlab.io/python-qemu-qmp/
- Revamps some of the docstrings to make the rendered output more
appealing/accurate/etc.

I'm looking for holistic feedback on the documentation. Feedback on
grammar, readability, layout and presentation of the HTML docs, etc
are all welcome. If you have feedback on the design of the library
itself: that is also extremely welcome, but may need to be addressed
in subsequent MRs, filed as issues/bugs/change-requests and so on.

Note: This MR is going to show a failed pipeline. This is due to other
GitLab configuration issues that I'm working on addressing in !3.
However, I wanted to begin soliciting feedback on the documentation
here in the meantime, because I predict it will be a very iterative
process.