rapper: add page (#3059)

pages/common/rapper.md

@@ -0,0 +1,13 @@
+# rapper
+
+> The Raptor RDF parsing utility.
+> Part of the Raptor RDF Syntax Library.
+> More information: <http://librdf.org/raptor/rapper.html>.
+
+- Convert an RDF/XML document to Turtle:
+
+`rapper -i rdfxml -o turtle {{file}}`
+
+- Count the number of triples in a Turtle file:
+
+`rapper -i turtle -c {{file}}`