arc: add page (#3430)

pages/common/arc.md

@@ -0,0 +1,20 @@
+# arc
+
+> Arcanist: A CLI for Phabricator.
+> More information <https://secure.phabricator.com/book/phabricator/article/arcanist/>.
+
+- Send the changes to Differential for review:
+
+`arc diff`
+
+- Show pending revision information:
+
+`arc list`
+
+- Update Git commit messages after review:
+
+`arc amend`
+
+- Push Git changes:
+
+`arc land`