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>
diff --git a/scripts/requirements-doc.txt b/scripts/requirements-doc.txt
index 8400552..b11f809 100644
--- a/scripts/requirements-doc.txt
+++ b/scripts/requirements-doc.txt
@@ -8,5 +8,6 @@
 pygments~=2.9
 sphinx-notfound-page>=0.6
 
-# Used by zephyr_module
+# YAML validation. Used by zephyr_module.
+PyYAML>=5.1
 pykwalify