Add generated documentation

[elliot-100]

Coming back to this project after some time, I'm finding it a bit difficult to follow the existing documentation and think it would be good to publish API docs from docstrings, instead of the hand-written notes in README.

pdoc is probably the simplest - I have it working for Spond-classes, published to GitHub Pages.

I'll try to get it working in a branch as as proof-of-concept, to the extent developers can browse a local version.

[elliot-100]

Done: https://github.com/Olen/Spond/tree/feat/auto-doc. Not intended to be merged for the time being.

Added a note in the README on how to invoke, but: poetry run pdoc spond