Updates for ReadTheDocs documentation

[juhoinkinen]

Miscellaneous updates and tweaks for the documentation, prompted by the email from RTD:

We are removing the "use system packages" feature on August 29th. Make sure you are installing all the required dependecies to build your project's documentation using a requirements.txt file and specifying it in your .readthedocs.yaml.

It seemed were having the setting system_packages: true for no reason in the configuration, so it is removed.

Also

• updates the RTD build environment to Ubuntu 22.04 and Python 3.10
• adds SWTFSA dependency to be installed to have its docstrings in the docs (forgotten from Make stwfsapy an optional dependency #700)
• removes pycld3 dependency (forgotten from use Simplemma instead of pycld3 for language detection #626)
• refines and updates the documentation page (using tips from a blog post)

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