samples shell/fs docs: Enable for native_sim

Instead of always enabling fuse for native_posix,
let's give users instructions on how to enable it
while building.

In the documentation: replace the references to native_posix
with native_sim, improve the links, mention the flash
simulator, and provide a separate subsection for FUSE access.

Add a separate test for the sample with fuse access enabled
and be sure it is possible to run both this and the basic
one with native_sim (was filtered out before for native_posix)

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>