Document Postgres 16 ICU library not found error

What does this merge request do and why?

Adds a Troubleshooting section to Postgres doc, and adds a section to resolve an error while installing Postgres 16.

This is early since we haven't attempted to add PG 16 support to GDK yet. But this is very easy to remove if it becomes unnecessary. I just didn't want to forget about it if it ends up being useful.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

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.
• [-] Announcement added, if change is notable.
• [-] gdk doctor test added, if needed.
• [-] Add the ~highlight label if this MR should be included in the CHANGELOG.md.