Add example config and zones.txt files

Provide catz.yaml.example and zones.txt.example as starting points
for new users. The config follows RFC 9432's recommendation to use
"invalid." for SOA mname/rname. The zones file demonstrates all
supported features: single/multiple catalog assignment, group
property, coo property, and combinations.

catz.yaml.example

@@ -0,0 +1,17 @@
+# catalog-zone-gen configuration
+#
+# Each catalog becomes an output zone file named <zone>.zone
+# Catalog names here are referenced in zones.txt
+
+catalogs:
+  prod:
+    zone: catalog-prod.example.com.
+  staging:
+    zone: catalog-staging.example.com.
+
+# SOA fields for all generated zone files
+# RFC 9432 recommends "invalid." for both since catalog zones
+# are not queried via normal DNS resolution.
+soa:
+  mname: invalid.
+  rname: invalid.