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/tutorials/template-from-scratch.md

@@ -171,7 +171,7 @@ resource "coder_agent" "main" {
 Because Docker is running locally in the Coder server, there is no need to
 authenticate `coder_agent`. But if your `coder_agent` is running on a remote
 host, your template will need
-[authentication credentials](../admin/external-auth.md).
+[authentication credentials](../admin/external-auth/index.md).
 
 This template's agent also runs a startup script, sets environment variables,
 and provides metadata.
@@ -181,7 +181,7 @@ and provides metadata.
   - Installs [code-server](https://coder.com/docs/code-server), a browser-based
     [VS Code](https://code.visualstudio.com/) app that runs in the workspace.
 
-    We'll give users access to code-server through `coder_app`, later.
+    We'll give users access to code-server through `coder_app` later.
 
 - [`env` block](https://registry.terraform.io/providers/coder/coder/latest/docs/resources/agent#env)