Update installation documentation to match Poetry2

The documentation for the installation is misleading and confusing, mixing old venv/pip install and poetry.

This is a pitty, as the new poetry way of install works like a charm when you get to understand its logic.

I would suggest the following process :

As root, create the poetry2 environment for www-data :

cd /var/www
for dn in .cache .local misp-modules; do
sudo mkdir /var/www${dn}
sudo chown www-data:www-data /var/www/${dn}
done

Switch to www-data user with sudo -u www-data -s

Download Poetry2 and install it in www-data 's environment (don't use system-suplied poetry, likely outdated).

curl -sSL https://install.python-poetry.org | python3 -
export PATH="/var/www/.local/bin:$PATH"
poetry --version

Download misp-modules and install it :

cd /var/www
git clone https://github.com/MISP/misp-modules.git
poetry install -E all

Get path to poetry's venv :

poetry env info --path

Use the poetry venv path to create the systemd file in /etc/systemd/system/misp-modules.service:

[Unit]
Description=System-wide instance of the MISP Modules
After=network.target

[Service]
User=www-data
Group=www-data
WorkingDirectory=/var/www/misp-modules
ExecStart=/var/www/.cache/pypoetry/virtualenvs/misp-modules-a47tzaLC-py3.12/bin/misp-modules -l 127.0.0.1

[Install]
WantedBy=multi-user.target

Locate the installation documentation for misp-modules; no file or test is named in the issue. Compare its current venv/pip and Poetry instructions with the proposed Poetry 2 workflow, then verify that the documented commands, environment paths, and systemd service setup are consistent.