Documentation: update install instructions with postGIS extension creation

[ghost]

The solution to this issue #582 should be added to the install page of the website documentation, I had to come search in (closed) issues to solve the same issue.

[cappert]

It's on https://postgis.net/install/:

PostGIS is an optional extension that must be enabled in each database you want to use it in before you can use it. Installing the software is just the first step.