docs: Add link to ignore (thanos-io#7926)

Signed-off-by: Saswata Mukherjee <[email protected]>

Author: saswatamcode

.mdox.validate.yaml

@@ -47,3 +47,6 @@ validators:
   # Expired certificate
   - regex: 'bestpractices\.coreinfrastructure\.org\/projects\/3048'
     type: 'ignore'
+  # Frequent DNS issues. 
+  - regex: 'build\.thebeat\.co'
+    type: 'ignore'