docs(upgrade): Add documentation on upgrade and qgis-plugin update

[plduthoit]

• Ticket : fix The lizmap_server plugin needs to be updated #68
• Add documentation, to fix issue The lizmap_server plugin needs to be updated #68, to provide documentation on how to upgrade lizmap and services to newer versions, and to handle qgis plugins update as it could break upgrades

[nworr]

Looks good to me.

Waiting for another reviewer before merging

thanks @plduthoit

[Gustry]

Suggested change

	docker compose exec map bash
	docker compose exec map /src/install-lizmap-plugin.sh
	docker compose restart map

See my comment in the issue #68 if this can be simplified. Doing multiple commands using exec is not convenient.

Or even from your host

./configure.sh configure

which is already in the readme.