Add (now mandatory) .readthedocs.yaml file, add docs extras and update sphinx conf

[misl6]

readthedocs.org now requires a v2 configuration file, in order to build the docs.

Unfortunately, for this reason, readthedocs was unable to build docs for latest 1.6.0 pyjnius release. (so a 1.6.1 will be released just after this PR is merged)

This PR does the following:

• Adds a .readthedocs.yaml file, with the proper configuration.
• Adds a requirements.txt file into docs folder, so readthedocs.org can use that to install deps.
• Updates the theme to furo, a nice and clean theme, also used by pip.
• Takes the pyjnius version from pyjnius, by reading it on __init__.py file (we never updated that previously, so manual is not an option 😀, but we can't install pyjnius)
• Now the copyright year is updated automatically (again: we never updated that previously, so manual is not an option 😀)

For PR reviewer, the newly rendered docs are available here: https://pyjnius.readthedocs.io/en/feat-add-readthedocs-file/

[misl6]

Update: Moved back in WIP, as while doing the same changes for pyobjus, I needed to follow another approach (as we can't install pyobjus on readthedocs), and would be nice to use the same.

[misl6]

Update: Now readthedocs.org can process the docs build without installing pyjnius (and its dependencies)

[cmacdonald]

lgtm

[AndreMiras]

Thanks for investigating into this