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.

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 pyobjus version from pyobjus, by reading it on __init__.py file (we never updated that previously, so manual is not an option 😀, but we can't install pyobjus)
• Now the copyright year is updated automatically (again: we never updated that previously, so manual is not an option 😀)

Similar to: kivy/pyjnius#684