Add GitHub Pages verification TXT for sid.is-a.dev

Adds a DNS TXT record required by GitHub Pages to release the custom-domain
lock on sid.is-a.dev so it can be assigned to 11sid11/11sid11.github.io.

The verification code is obtained from the GitHub Pages domain verification
flow (https://github.com/settings/pages).

Per the is-a-dev dnsconfig.js schema, this is a separate domain file at the
verification hostname path, following the same pattern as other verified
subdomains.

domains/_github-pages-challenge-11sid11.sid.json

@@ -0,0 +1,9 @@
+{
+  "owner": {
+    "username": "11sid11",
+    "email": "sid6117828@gmail.com"
+  },
+  "records": {
+    "TXT": "3ee5f5a1c7920d09e2e2cf4699bbc2"
+  }
+}