Add GitHub Pages verification TXT record as its own nested subdomain file

Per maintainer feedback on the previous PR, this belongs in its own
file (domains/_github-pages-challenge-rustyisacat.rusty.json) rather
than merged into domains/rusty.json.

domains/_github-pages-challenge-rustyisacat.rusty.json

@@ -0,0 +1,9 @@
+{
+  "owner": {
+    "username": "rustyisacat",
+    "email": "harrisonchaffin@gmail.com"
+  },
+  "records": {
+    "TXT": "29a3a85a3f99167cb05833fe0b5d30"
+  }
+}