Add .readthedocs.yaml

[benjaoming]

Currently, our docs aren't building because we haven't configured anything. By default, Read the Docs uses old legacy tools when nothing is configured -- and that doesn't work.

Example: https://readthedocs.org/projects/django-nyt/builds/20372734/

[oscarmcm]

This might be what we need?

version: 2
build:
  os: ubuntu-20.04
  tools:
    python: "3.9"
sphinx:
   configuration: docs/conf.py
python:
   install:
    - method: pip
      path: .
      extra_requirements:
        - docs

[benjaoming]

Yes, I think that's good :+1: