Documentation not building

[mpmuc84]

I noticed the online documentation has failed to build for more than 1.5 years (more specifically: since RTD deprecated their configuration file specifications, cf. readthedocs/readthedocs.org#10367) due to the deprecated configuration file for djangosaml2.

The missing updates are not many, but most notably instructions on security and CSP are missing, and for future updates it would be nice to have current documentation available via RTD.

Would you care for a PR to fix this or are you planning to update the configuration already?

[peppelinux]

@mpmuc84 please create a PR to fix this issue

very appreciated, thank you