identity: CIMD client registration (w. UI)

## Jobs to be done (optional)

Document MCP registration via CIMD (Client Metadata Documents), where a OAuth client uses an HTTPS URL as `client_id` and fetches the client's metadata from that URL.

## Definition of done

- Update auth servers page
- How-to guides (for (auth server admin):
- [ ] Configure a single upstream OIDC IdP connection to an auth server
- [ ] Revoke a consent grant

## Information

[PRD and use cases](https://docs.google.com/document/d/16F4jqItK4S5HVPw5gdSJDGxWcb5Xypq_rtQ1wqPpXw0/edit?tab=t.y0w6iuf2wgtq#heading=h.dwjnhdg0984f)

## Due date (optional)

## Size

Start by reviewing the auth servers page and the linked PRD and use cases to determine the intended CIMD registration and UI scope. Document MCP registration, update the auth servers page, and add the two requested auth server administrator how-to guides; completion is marked by those documentation updates being present.

Written by the indexing model from the issue text.