Authorization docs finetuning

[soxofaan]

This is an epic to list tickets, todo's and ideas on further finetuning of auth related docs

• Deprecate OIDC Authorization code grant support and usage #235
• Refer to openeo.org Auth introduction #154 and related PR Issue154 Refer to openeo.org in auth doc #253
• Reduce importance of AuthConfig (for OIDC) #290
• more discussion of client credentials and pointers to more concrete instructions for specific back-ends

[soxofaan]

Related: in eu-cdse/documentation#613 I'm working on authentication related docs for CDSE (especially machine-to-machine use cases). I think some parts of that might also belong in the general python docs

edit: had to re-do the PR -> eu-cdse/documentation#624