Running `just add-inventory-host example.com 1.2.3.4` (or
`make add-inventory-host domain=example.com ip=1.2.3.4`) adds a new
host to the inventory, creating inventory/hosts and
inventory/host_vars/matrix.DOMAIN/vars.yml from the example files,
with strong secrets generated automatically.

Existing configuration is never overwritten. The command refuses to
run if the host is already in the inventory, so it can also be used
for adding more hosts later.

Based on the idea proposed in
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/4682 by @Ser5

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>