Refresh OIDC Documentation

[ravindk89]

Summary

Reusing an older issue to cover:

• Generally refresh OIDC documentation
• Add new environment variables
• Add procedure for KeyCloak + AdminAPI extension
• Add procedure for Dex
• Add policy variable details (include AD/LDAP as well)

Stretch goal: Basic docs on the plug-in IDP

[djwfyi]

Will not cover Dex until asked.