Add documentation on how to free disk space

[lucamanga]

Checklist

• [x] I've searched through the current issues to make sure this feature hasn't been requested already.

Motivation

I noticed that, over time, G3WSuite creates many temporary files. Here are some "big" folders I found:

1. In /tmp there are many files like this (I'm using the docker version)

   QGIS3-xxxxx  (as folder)
   QGIS3.xxxxx  (as file)

2. in .../media/temp_upload I have 3.221 qgs/qgz files, for a total directory size of 3.3GB

Suggested solution

In the documentation, suggest a script to clean files (to run via crontab on night)

Alternatives considered

No response