Installation and Setup documentation is outdated

[The Install PostGIS and system packages section of the documentation](https://docs.ckan.org/projects/ckanext-spatial/en/latest/install.html#install-postgis-and-system-packages) is outdated, covering only Ubuntu 12.04 and 14.04.

Starting from PostgreSQL 9.2, there is [a simpler way of enabling PostGIS than directly running the SQL files](https://postgis.net/docs/postgis_installation.html#create_new_db_extensions).

Note that the "Install the extension" step's name is ambiguous, since PostGIS is also an extension. It would be clearer to name it "Install the extension of CKAN".

Start with the linked “Install PostGIS and system packages” section in the CKAN spatial documentation and compare its instructions with the linked PostGIS installation guidance. Update the outdated Ubuntu coverage, document the simpler PostgreSQL extension method, and rename “Install the extension” to clarify that it refers to the CKAN extension.

Written by the indexing model from the issue text.