Documentation - Proposed changes for Ubuntu (venv) installation >=23.04

[glenm-nz]

Since Ubuntu 23.04 (approx?), they have moved to externally managed environments, with the python packages sourced via apt.

Unfortunately many of the dependencies for mqttany are not available in these apt packages, and still need to be installed via PIP.

For the installation documentation https://crazyivan359.github.io/mqttany/start.html#installation, I would recommend splitting section 3 into two options, the first being the installation as currently described, the second being a 'Python virtual environment' installation for Ubuntu etc, e.g:

sudo python3 -m venv /opt/mqttany/mqttany/venv
sudo /opt/mqttany/mqttany/venv/bin/pip3 install -r /opt/mqttany/requirements/mqttany.txt
sudo /opt/mqttany/mqttany/venv/bin/pip3 install -r /opt/mqttany/requirements/mqtt.txt

This would also need a note on this page: https://crazyivan359.github.io/mqttany/launch.html to modify /etc/systemd/system/mqttany.service for the following like to read:

ExecStart=/opt/mqttany/mqttany/venv/bin/python3 mqttany.py

Hopefully this will help save some users time/questions. I did the above with Python 3.12 and following these instructions works fine (assuming #162 is resolved, and no manual edits of the requirements.txt would therefore be needed)

Cheers