Authorization docs finetuning

[soxofaan]

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

• https://github.com/Open-EO/openeo-python-client/issues/235
• https://github.com/Open-EO/openeo-python-client/issues/154 and related PR https://github.com/Open-EO/openeo-python-client/pull/253
• https://github.com/Open-EO/openeo-python-client/issues/290
• more discussion of client credentials and pointers to more concrete instructions for specific back-ends

[soxofaan]

Related: in https://github.com/eu-cdse/documentation/pull/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 -> https://github.com/eu-cdse/documentation/pull/624