Try to exclude the 'onlinedocs' branch from actions

There's no sourcecode within that branch, anyway.

Author: dl8dtl

.github/workflows/build.yml

@@ -20,8 +20,14 @@ name: Build
 
 on:
   push:
+    branches-ignore:
+      - 'onlinedocs'
   pull_request:
+    branches-ignore:
+      - 'onlinedocs'
   workflow_call:
+    branches-ignore:
+      - 'onlinedocs'
 
 env:
   BUILD_TYPE: RelWithDebInfo