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)