chore(docs): Trust Self Signed Certificates with Immich And OAuth Server

[seifer44]

Description

Added an additional guide to setting up a private CA trust with a local OAuth server.

How Has This Been Tested?

1. Deploy an Authentik server (such as with Docker Compose).
2. Set up an NGINX server as a reverse proxy for Authentik, using a self-signed HTTPS certificate.
3. Establish OAuth Authentication with Immich with your local Authentik server. Use HTTPS through the reverse proxy NGINX.

More information in the discussion #18614

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if applicable
• I have no unrelated changes in the PR.
• I have confirmed that any new dependencies are strictly necessary.
• I have written tests for new code (if applicable)
• I have followed naming conventions/patterns in the surrounding code
• All code in src/services/ uses repositories implementations for database calls, filesystem operations, etc.
• All code in src/repositories/ is pretty basic/simple and does not have any immich specific logic (that belongs in src/services/)