yard: add page (#10486)

* yard: add page

pages/common/yard.md

@@ -0,0 +1,16 @@
+# yard
+
+> Documentation tool for Ruby.
+> More information: <https://yardoc.org/>.
+
+- Create the documentation:
+
+`yard`
+
+- Create the documentation and save it to one file:
+
+`yard --one-file`
+
+- List all undocumented objects:
+
+`yard stats --list-undoc`