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 recently.

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 plyer version from plyer, by reading it on __init__.py file, as we can't import plyer on readthedocs
• Now the copyright year is updated automatically

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