docs: harden security for debian install

jooolarequested to merge
jooola/funkwhale:docs_debian_security into develop

Fix #1826 (closed)

Funkwhale only need to read the source/config files and read/write to the data directories.

The source/config files manipulation must be handled by a privileged user.

cc @JuniorJPDJ @georgkrause

Edited by jooola

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