Added solution for missing python deps when running as service

[potatoes1286]

Summary

As i was installing this as a service on Ubuntu service 20.04.5 LTS, i came across three issues:

1. /venv/ in service file was incorrectly written as /env/
2. user was predefined as brecht, which would cause the service to fail as user did not exist (may vary by machine)
3. service was unable to access installed python dependencies from the virtual environment and user space- and required installing from superuser

Details

1. renamed /env/ to /venv/
2. user entry was changed to <user> for the end user to change
3. added instructions on how to install requirements system-wide should the service not be able to access the virtual environment installed requirements

Checks

• In case of new feature, add short overview in docs/<corresponding file>
• Tested changes