ci: add comment with preview theme workflow disable reason (#4597)

Co-authored-by: Alexandr <qwerty541zxc@gmail.com>

Author: alexandr-garbuzov

.github/workflows/preview-theme.yml

@@ -1,5 +1,7 @@
 name: Theme preview
 on:
+  # Temporary disabled due to paused themes addition.
+  # See: https://github.com/anuraghazra/github-readme-stats/issues/3404
   # pull_request_target:
   #   types: [opened, edited, reopened, synchronize]
   #   branches: