[irods/irods#7274] Add authentication configuration docs

[alanking]

In service of irods/irods#7274

Still working on the issue itself (tests are finding serious problems) but figured I would go ahead and put this up for eyeballs on the words.

[alanking]

Roping in irods/irods#7357 and irods/irods#7358

[trel]

Remove GSI and Kerberos in separate commits please.

[alanking]

Hmm... okay, how do I split removing the sentence "GSI and Kerberos are also available."? There were a couple of instances like that and it wasn't clear to me how to split that up.

[trel]

Oh - yeah that’s not as obvious. Yeah pick one for that sentence.

[alanking]

When squashed, the desired effect will occur. Please let me know when we're ready for a force-push

[trel]

go for it

[alanking]

Pushed

[trel]

Yeps. Go for it.