doc-build.yml: add top-level Makefile

Dunno why the top-level Makefile is doc/ specific but it is.

Signed-off-by: Marc Herbert <[email protected]>

Author: marc-hb

.github/workflows/doc-build.yml

@@ -6,6 +6,7 @@ name: Documentation GitHub Workflow
 on:
   pull_request:
     paths:
+    - 'Makefile'
     - 'doc/**'
     - '**.rst'
     - 'include/**'