Doc updates for new ReadTheDocs requirements

[justb4]

Is your feature request related to a problem? Please describe.
RTD has some new policies, and also the API docs (Python module APIs) are not rendered by RTD.
Webhook seems broken. Can only be tested/verified by direct push on master.

Describe the solution you'd like
Add a requirements.txt for docs and update .readthedocs.yaml

Describe alternatives you've considered
Not really

Additional context
N.A.

[justb4]

API docs generated in https://docs.geohealthcheck.org/en/latest/autoapi/ but in plugins.rst automodule does not work. Should we fix or just link to autoapi sections?

Plugins - Base Classes
......................

These are the base classes for GHC Plugins. Developers will
mainly extend `Probe` and `Check`.

.. automodule:: GeoHealthCheck.plugin
   :members:
   :show-inheritance: