Create docs for OIDC/OAuth test setup

What does this MR do and why?

As per #404920 (closed), we are adding documentation on How to test OIDC/OAuth on GitLab.

Screenshots or screen recordings

How to set up and validate locally

1. Checkout the branch 404920-create-docs-for-oidc-oauth-test-setup
2. Visit the documentation page (http://gdk.test:3000/help/api/oidc_oauth.md)
3. Validate that page is existing and working

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #404920 (closed)

doc/administration/auth/index.md

@@ -33,3 +33,7 @@ For more information, see the links shown on this page for each external provide
 | **User Removal**                                | SCIM (remove user from top-level group) | LDAP (remove user from groups and block from the instance)<br>SCIM |
 
 1. Using Just-In-Time (JIT) provisioning, user accounts are created when the user first signs in.
+
+## Test OIDC/OAuth in GitLab
+
+See [Test OIDC/OAuth in GitLab](test_oidc_oauth.md) to learn how to test OIDC/OAuth authentication in your GitLab instance using your client application.