.github/workflows: Don't run the sanity jobs for markdown-only updates (#2356)

Update the sanity workflow and avoid running the defined jobs when only
markdown changes are present in the PR. This is a follow-up to #2347
which enabled this behavior with the other actions, and this sanity
action was introduced after that PR had already merged.

Signed-off-by: timflannagan <[email protected]>

Author: timflannagan

Diff for: .github/workflows/sanity.yaml

@@ -5,9 +5,8 @@ on:
     branches:
       - '**'
   pull_request:
-    paths:
-      - '**'
-      - '!doc/**'
+    paths-ignore:
+    - '**/*.md'
 
 jobs:
   sanity: