Move Environment variables infos to manual

[fkaminski]

As discussed on Slack.

[fkaminski]

Maybe we should then remove the information from the dev doc and link to the manual instead.

[fkaminski]

Environment variables can also be defined on the system level - globally, per user or per process.

@fritzmg As mentioned i could need help on this. On the other hand, since this is part of the manual, isn't the information about .env/.env.local sufficient?

[fkaminski]

Thank you Bjarke