Document proj network grids missing

[BWibo]

Document behavior due to missing grid files in proj and how to mitigate this.
See here for more: https://blog.cleverelephant.ca/2023/02/proj-network.html

It should be explicitly stated that not including the grid files may lead to wrong coordinate transformations, for example, when preparing 3DCityDB data contents for 3D visualization.

ToDos

• Document for Docker images using env var PROJ_NETWORK=ON -> Docker image usage
• Document for pare metal PostgreSQL installations PROJ_NETWORK=ON -> Manual 3DCityDB setup, proj.ini (/usr/share/proj)