Add developer documentation for debugging GitLab with VS Code

What does this MR do and why?

This MR adds a documentation page called VS Code Debugging. It explains how to set up the VS Code editor for debugging the main GitLab Rails app.

All credit for the configuration described in this MR goes to @zillemarco 🙌. He designed this setup and described it in this slack thread: https://gitlab.slack.com/archives/CLX2Z53A5/p1679518369034109