Highlight common first time install issues

What does this merge request do and why?

Context: I am a new GitLab engineer installing GDK for the first time. I'm proposing a couple changes that I believe could help GDK users in the future.

Added section under Gitaly troubleshooting for "socket path too long" error.

Added a small note about pulling gitlab repos into working directory, just to avoid confusion about where GDK looks for those repos in the dev environment.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.