Fix python documentation warnings and format

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Description

This fixes the format of the Python documentation, and removes Sphinx and Pip warnings when building the documentation, including a warning that was only shown when using the GStreamer compatibility. It also allows to overwrite the Python version used by FindPython again (remove a FORCE on a set), which allows to make it work on a system where a Python version more recent than the system default Python version is installed.

References

Closes https://github.com/introlab/opentera-webrtc-ros/issues/69

Testing

[ ] Building this repo, including the building of the documentation and with the GStreamer support on a fresh Ubuntu 20.04 (no other Python packages installed except for dependencies of this package) should not produce Sphinx warnings or pip warnings anymore (red block of text)
[ ] The HTML documentation built should not include unformated :return: or :param, and should be free of C++ artifacts (there used to be a mention of opentera::IceServers, it should now be only IceServers as it should)
[ ] Building this repo via building https://github.com/introlab/opentera-webrtc-ros using catkin on a system where Python 3.9 or higher is also installed should now work and correctly use the system (and ROS) version of Python.

Checklist

[ ] (N.A.) I have added documentation for new/changed functionality in this PR
[x] I have formatted the files
[x] All active GitHub checks for tests are passing
[x] The correct base branch is being used, if not main

introlab / opentera-webrtc