requirements-doc: add PyYAML which removes dependency on -base

This means a light requirements-doc.txt is enough for doc writers. See
previous discussions in PR #31199 and PR #31239

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

.github/workflows/doc-build.yml

@@ -45,7 +45,6 @@ jobs:
     - name: install-pip
       run: |
         sudo pip3 install -U setuptools wheel pip
-        pip3 install -r scripts/requirements-base.txt
         pip3 install -r scripts/requirements-doc.txt
         pip3 install west==0.11.0
 

scripts/requirements-doc.txt

@@ -8,5 +8,6 @@ sphinxcontrib-svg2pdfconverter
 pygments~=2.9
 sphinx-notfound-page>=0.6
 
-# Used by zephyr_module
+# YAML validation. Used by zephyr_module.
+PyYAML>=5.1
 pykwalify