[doc] Arch Linux installation: modified the command for user creation so it works on both Debian and Arch

FW requires Python>=3.5 ; only Python 3.4 is packaged in Debian 8.

fix #300

Seems that the extensions are per-database.
Without that tweak, the install will fail at django migrations step, yelling for insuficient rights to install the accent extension itself.

Which will fail if the system does not contains this locale.

Without that, starting from a debian « naked » system,  it will fail at the `pip install wheel` stage.

rmdir will fail on that non-empty dir

run makemigrations yourself

Updated sample nginx, docker-compose.yml and documentation to match easier setup for front-end files