Documentation : How to create a demo data file

[ponceta]

We should document how to create a demo data file to be usable by pum in oqtopus :

For TWW, the list of commands are provided here :

https://github.com/teksi/wastewater/blame/main/datamodel/scripts/export_demo_data.sh

We should :

Have a template with this real life example

Document what and why these commands are necessary.

Document the configuration when a/several demo data file is/are provided.

This should be in the adding module section.

[3nids]

fixed in opengisch/pum#162
see https://opengisch.github.io/pum/configuration/demo_data/