Fix Read the Docs build

[pquentin]

The .readthedocs.yaml file is now required: https://blog.readthedocs.com/migrate-configuration-v2/

I faced two main issues here:

• Our config file was not asking for sphinx-rtd-theme on Read the Docs, which was useful long ago but prevented the theme to be used today. This is why https://elasticsearch-dsl.readthedocs.io/en/latest/ shows the default theme instead (alabaster).
• By default, Read the Docs installs old versions of sphinx and sphinx-rtd-theme, so I had to put lower bounds to avoid that.

[JoshMock]

Looks like a couple small CI failures, but everything looks good on the whole. 👍

[pquentin]

I have enabled the Read the Docs pull request integration so the result can be seen here: https://elasticsearch-dsl--1668.org.readthedocs.build/en/1668/. Please take another look as I have continued to tweak things to make the build work.