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/admin/templates/extending-templates/icons.md

@@ -32,7 +32,7 @@ come bundled with your Coder deployment.
   }
   ```
 
-- [**Authentication Providers**](https://coder.com/docs/admin/external-auth):
+- [**Authentication Providers**](../../external-auth/index.md):
 
   - Use icons for external authentication providers to make them recognizable.
     You can set an icon for each provider by setting the

docs/admin/templates/open-in-coder.md

@@ -15,7 +15,7 @@ approach for "Open in Coder" flows.
 
 ### 1. Set up git authentication
 
-See [External Authentication](../external-auth.md) to set up git authentication
+See [External Authentication](../external-auth/index.md) to set up Git authentication
 in your Coder deployment.
 
 ### 2. Modify your template to auto-clone repos