Docs: basic docs for SAML SSO

[stsewd]

We still need to figure out a couple of things, but I think it doesn't hurt to have some basic docs about this feature.

This follows the same structure from other guides related to SSO, preview at https://docs--11288.org.readthedocs.build/en/11288/guides/set-up-single-sign-on-saml.html.

Ref https://github.com/readthedocs/readthedocs.org/pull/11262

---

:books: Documentation previews :books:

• User's documentation (docs): https://docs--11288.org.readthedocs.build/en/11288/

• Developer's documentation (dev): https://dev--11288.org.readthedocs.build/en/11288/

[stsewd]

This documentation isn't complete, as the SAML feature isn't complete yet, but should be good enough to guide users that want to beta test it or for ourselves.

[cklim5]

This documentation isn't complete, as the SAML feature isn't complete yet, but should be good enough to guide users that want to beta test it or for ourselves.