SSO in Admin UI

[taroface]

SSO will be implemented for the Admin UI.

• Document new auth flow
• Update any existing sign-on procedures, if necessary

Roadmap issue: https://cockroachlabs.atlassian.net/browse/CRDB-211
config and permissions docs: https://docs.google.com/document/d/1K7X1OBqlOAMsy12NlEOKu_wxXpu6TXa53L3bFJZYiIE/edit#
how-to steps: cockroachdb/cockroach#52609

[jseldess]

2 related release notes:

PR: cockroachdb/cockroach#54659

Added ability to login via OIDC provider. [#54659][#54659] {% comment %}doc{% endcomment %}

Added ability to login via OIDC provider: Operators can now configure a single OIDC authentication provider using a collection of cluster settings that can provide authentication for users into the Admin UI only (not for SQL). The user must exist as a SQL user in the database for the authentication to work. [#54659][#54659] {% comment %}doc{% endcomment %}