This commit adds a configuration guide for the newly introduced
OpenID Connect realm. The guide is similar to the style of the
SAML Guide and shares certain parts where applicable (role mapping)
It also contains a short section on how the realm can be used for
authenticating users without Kibana.
Co-Authored-By: Lisa Cawley <lcawley@elastic.co>
Backport of #41423 and #42555
Ioannis Kakavas