Update troubleshooting docs to mention Postgres blocker
Based on #420 (closed), add note regarding gitlab-ce#54718 because it's a blocker for any other users trying to use the GDK.
@stanhu I figured it's a good idea to add this to the troubleshooting docs; so anyone who tries to use the GDK in the meantime can do so, because I actually spent 3 days thinking I had an issue on my end before I opened an issue here.
What do you think?
Edited by Omar Bahareth