Add .mdx extension and make frontmatter simpler

Author: RebeccaTamachiro

src/content/docs/dns/manage-dns-records/troubleshooting/acme-challenge-txt-record renamed to src/content/docs/dns/manage-dns-records/troubleshooting/acme-challenge-txt-record.mdx

@@ -3,10 +3,7 @@ title: Unexpected _acme-challenge TXT records
 pcx_content_type: troubleshooting
 sidebar:
   order: 11
-head:
-  - tag: title
-    content: Unexpected _acme-challenge TXT records are present
-
+  label: Unexpected TXT records
 ---
 
 import { Render } from "~/components"