feat: add GitHub Pages challenge TXT record for phoenix.is-a.dev

Registers _github-pages-challenge-mj-praba.phoenix.is-a.dev as a TXT
record to verify the phoenix.is-a.dev custom domain in GitHub Pages
settings.

domains/_github-pages-challenge-mj-praba.phoenix.json

@@ -0,0 +1,9 @@
+{
+    "owner": {
+        "username": "mj-praba",
+        "email": "manojprabaharj@gmail.com"
+    },
+    "records": {
+        "TXT": "5452a491b95bcdd3a1c52cf415670c"
+    }
+}