Add (now mandatory) .readthedocs.yaml file, add docs requirements.txt 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 python-for-android.

This PR does the following:

• Adds a .readthedocs.yaml file, with the proper configuration.
• Adds a requirements.txt file into doc 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 python-for-android version from python-for-android, by reading it on __init__.py file (we never updated that previously, so manual is not an option 😀, but we can't install python-for-android)
• 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://python-for-android.readthedocs.io/en/feat-add-readthedocs-file