Add ag (the_silver_searcher) page

pages/common/ag.md

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+# ag
+
+> The Silver Searcher. Like ack, but faster.
+
+- Find files containing "foo"
+
+`ag foo`
+
+- Find "foo" in files with a name matching "bar"
+
+`ag foo -G bar`
+
+- Find files whose contents match a regular expression:
+
+`ag '^ba(r|z)$'`
+
+- Find files with a name matching "foo"
+
+`ag -g foo`