Refactor inventory into OS groups and update playbooks accordingly

Group hosts by OS (ubuntu/rocky) in inventory, refactor playbooks to
use group-targeted plays instead of ansible_os_family conditionals,
add bot host, and add Docker volume backup playbook via rsync to storage.

inventories/all.yml

@@ -1,20 +1,20 @@
 all:
-  hosts:
-    webserver:
-      ansible_host: 192.168.2.15
-      ansible_user: root
-    nextcloud:
-      ansible_host: 192.168.2.120
-      ansible_user: root
-    pihole:
-      ansible_host: 192.168.2.2
-      ansible_user: root
-    docker:
-      ansible_host: 192.168.2.10
-      ansible_user: root
-    # eve:
-    #   ansible_host: 192.168.2.12
-    #   ansible_user: root
-    storage:
-      ansible_host: 192.168.2.115
-      ansible_user: root
+  vars:
+    ansible_user: root
+  children:
+    ubuntu:
+      hosts:
+        webserver:
+          ansible_host: 192.168.2.15
+        docker:
+          ansible_host: 192.168.2.10
+    rocky:
+      hosts:
+        pihole:
+          ansible_host: 192.168.2.2
+        bot:
+          ansible_host: 192.168.2.174
+        storage:
+          ansible_host: 192.168.2.115
+        nextcloud:
+          ansible_host: 192.168.2.120