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.

zones.txt.example

@@ -0,0 +1,23 @@
+# Zones to include in catalog zones
+#
+# Format:  <zone>  <catalog>[, <catalog>...]  [, group=<value>]  [, coo=<fqdn>]
+#
+# - Catalogs are bare names matching keys in catz.yaml
+# - group: RFC 9432 property — consumers apply shared config to grouped zones
+# - coo:   RFC 9432 change-of-ownership — points to the old catalog during migration
+
+# Simple: zone in one catalog
+app.example.com          prod
+
+# Zone served by multiple catalogs
+example.com              prod, staging
+
+# Group property — lets consumers apply per-group policy
+api.example.com          prod, group=critical
+dev.example.net          staging, group=testing
+
+# Change-of-ownership — zone migrating from another catalog
+legacy.example.org       prod, coo=old-catalog.example.com.
+
+# Both properties
+internal.example.org     prod, group=internal, coo=old-catalog.example.com.