ci: trigger doc build if requirements change

If Python requirements for docs change build should also be triggered.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/.github/workflows/doc-build.yml b/.github/workflows/doc-build.yml
index df014d0..7c17b2e 100644
--- a/.github/workflows/doc-build.yml
+++ b/.github/workflows/doc-build.yml
@@ -19,6 +19,7 @@
     - 'west.yml'
     - '.github/workflows/doc-build.yml'
     - 'scripts/dts/**'
+    - 'scripts/requirements-doc.txt'
 
 jobs:
   doc-build: