Add app_links Vale rule to flag private internal URLs (#89)

* Add app_links Vale rule to flag private internal Datadog URLs

Catches internal subdomain patterns (dd*.datadoghq.com, demo.datadoghq.com)
and datad0g.com links, suggesting app.datadoghq.com as the replacement.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

* Catch app.datadog.com and suggest app.datadoghq.com

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

---------

Co-authored-by: Claude Sonnet 4.6 <[email protected]>

Author: janine-c

styles/Datadog/app_links.yml

@@ -0,0 +1,16 @@
+# This file is used to lint links to internal Datadog environments that are not accessible to customers.
+extends: substitution
+message: "This looks like a private app link. Use '%s' instead of '%s'."
+link: "https://github.com/DataDog/documentation/blob/master/CONTRIBUTING.md#links"
+ignorecase: true
+level: error
+scope: raw
+action:
+  name: replace
+swap:
+  # Matches known internal subdomain patterns on the product domain
+  'https?://(?:dd[a-zA-Z0-9\-]*|demo)\.datadoghq\.com': https://app.datadoghq.com
+  # Matches the internal zero-substitution domain variant
+  'https?://(?:[a-zA-Z0-9][a-zA-Z0-9\-]*\.)?datad0g\.com': https://app.datadoghq.com
+  # Matches app.datadog.com (missing 'hq')
+  'https?://app\.datadog\.com': https://app.datadoghq.com