CI: Require doc team signoff for doc changes

Require an additional approval from a `documentation` team member for
PRs containing documentation changes.

Fixes #41.

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

.pullapprove.yml

@@ -30,3 +30,14 @@ groups:
     required: 2
     teams:
       - builder
+
+  documentation:
+    required: 1
+    teams:
+      - documentation
+    conditions:
+      files:
+        include:
+          - "*.md"
+        exclude:
+          - "vendor/*"