New error 500 page for users who want to troubleshoot

### What needs to be documented?

Add error 500 page to docs that allows users to understand how to troubleshoot their issues if they've received an error.

### Additional context

- https://platformsh.slack.com/archives/C088NDJ709Y/p1744835966601959 
- https://linear.app/platformsh/issue/FIRST-101/implement-custom-502-error-page-for-upsun#comment-1f29a057