docs: add more specific steps and information about oidc refresh tokens (#18336)

closes https://github.com/coder/coder/issues/18307

relates to https://github.com/coder/coder/pull/18318

preview:
-
[refresh-tokens](https://coder.com/docs/@18307-refresh-tokens/admin/users/oidc-auth/refresh-tokens)
-
[configuring-okta](https://coder.com/docs/@18307-refresh-tokens/tutorials/configuring-okta)
~(not sure why @Emyrk 's photo is so huge there though)~ ✔️
- [x] removed from
[idp-sync](https://coder.com/docs/@18307-refresh-tokens/admin/users/idp-sync)

to do:
- move keycloak
- add ping federate and azure
- edit text (possibly placeholders for now - I want to see how it all
relates and edit it again. right now, there's a note about the same
thing in every section in way that's not super helpful/necessary)
- ~convert some paragraphs to OL~ calling this out of scope for now

---------

Co-authored-by: EdwardAngert <17991901+EdwardAngert@users.noreply.github.com>

docs/ai-coder/best-practices.md

@@ -33,7 +33,7 @@ for development. With AI Agents, this is no exception.
   development without manual intervention (e.g. repos are cloned, dependencies
   are built, secrets are added/mocked, etc.).
 
-  > Note: [External authentication](../admin/external-auth.md) can be helpful
+  > Note: [External authentication](../admin/external-auth/index.md) can be helpful
   > to authenticate with third-party services such as GitHub or JFrog.
 
 - Give your agent the proper tools via MCP to interact with your codebase and