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/setup/appearance.md

@@ -41,7 +41,7 @@ users of which network their Coder deployment is on.
 
 ## OIDC Login Button Customization
 
-[Use environment variables to customize](../users/oidc-auth.md#oidc-login-customization)
+[Use environment variables to customize](../users/oidc-auth/index.md#oidc-login-customization)
 the text and icon on the OIDC button on the Sign In page.
 
 ## Support Links

docs/admin/setup/index.md

@@ -148,7 +148,7 @@ integrations with Git providers, such as GitHub, GitLab, and Bitbucket.
 External authentication can also be used to integrate with external services
 like JFrog Artifactory and others.
 
-Please refer to the [external authentication](../external-auth.md) section for
+Please refer to the [external authentication](../external-auth/index.md) section for
 more information.
 
 ## Up Next