Let Docker allocate the network subnet again

Reverts the explicit subnets: the pools were freed by deleting five networks, so
the network role is back to its single idempotent task - ensure the network
exists and Caddy is on it - and the Sites no longer declare a subnet. The
deployment readme keeps the note that a new Site needs a config, and therefore a
new tag, but no longer asks for a free /24.

If the pools fill up again the failure is recognisable: 'all predefined address
pools have been fully subnetted', and the choices are deleting stale networks or
declaring a subnet per Site.

ansible/vars/default.yml

@@ -8,9 +8,6 @@ site: '{{ inventory_hostname }}'
 image_name: registry.tanshu.com/menu-admin
 image_tag: "{{ tag | default('latest') }}"
 
-# Every Site sets network_subnet: the host's Docker address pools are exhausted,
-# so a network created without an explicit subnet is refused. Use a free /24 from
-# 10.210.0.0/16, one per Site.
 app_uid: 1000
 app_gid: 1000