gatus: alert through self-hosted ntfy

Add ntfy at ntfy2.rubenhensen.nl on the home cluster, pointed straight at
the home IP like s2. s2 now alerts on mail and on s1 through it. The
home services are left to s1, which sees them from outside.

ntfy denies everything except the provisioned user. The user, its
password hash and the Gatus token come from Vault kv/ntfy.

dns/domains/rubenhensen.nl.yaml

@@ -73,6 +73,15 @@ records:
     expire: 300
     type: A
     content: "62.41.86.27"
+  # ntfy for Gatus alerts, one next to each Gatus. Same reasoning as s2.
+  - name: "ntfy1"
+    expire: 300
+    type: A
+    content: "46.224.26.65"
+  - name: "ntfy2"
+    expire: 300
+    type: A
+    content: "62.41.86.27"
   - name: "@"
     expire: 300
     type: MX