[SYSTEMDS-3053] Do not run Action workflow for doc changes

companion to https://github.com/apache/systemds/commit/bae1506443b9c9576716291fc3cd72552a1359b8
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index d0ed51b..a5bb3dc 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -3,7 +3,11 @@
 on:
   push:
     branches: [ master ]
+    paths-ignore:
+      - '*.md'
   pull_request:
+    paths-ignore:
+      - '*.md'
     branches: [ master ]
   # enable manual workflow trigger
   workflow_dispatch: