Docs/sphinx html docs

[rdnfn]

This is a quick PR that add the up to date documentation configuration to the main branch.

Docs are available here: https://geograph.readthedocs.io/en/latest/

Main changes:

• Update rst files (that define which modules are shown in docs) to new repo structure
• Create doc requirements (run by readthedocs)

Other changes:

• Updated docstrings in some packages to comply with requirements for sphinx
• Added documentation to main readme

EDIT: it looks like quite a few files changed, but these are mainly doc specific rst files.

[rdnfn]

Looks good! One question: if we update the main branch after this PR is merged then will the docs update automatically? Or is there a way to do this in the future, if not?

Thanks! At the moment I trigger the docs builds manually, but in the future I should be able to add a webhook for it.

EDIT: there is a webhook now that should trigger it automatically 🚀