comby: add page (#7847)

pages/common/comby.md

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+# comby
+
+> Tool for structural code search and replace that supports many languages.
+> More information: <https://github.com/comby-tools/comby>.
+
+- Match and rewrite templates, and print changes:
+
+`comby '{{assert_eq!(:[a], :[b])}}' '{{assert_eq!(:[b], :[a])}}' {{.rs}}`
+
+- Match and rewrite with rewrite properties:
+
+`comby '{{assert_eq!(:[a], :[b])}}' '{{assert_eq!(:[b].Capitalize, :[a])}}' {{.rs}}`
+
+- Match and rewrite in-place:
+
+`comby -in-place '{{match_pattern}}' '{{rewrite_pattern}}'`
+
+- Only perform matching and print matches:
+
+`comby -match-only '{{match_pattern}}' ""`