zcmp: add page (#8599)

* zcmp: add page

* move zcmp.md and fix file paths

* clarify description

* Apply suggestions from code review

pages/common/zcmp.md

@@ -0,0 +1,12 @@
+# zcmp
+
+> Compare compressed files.
+> More information: <https://manned.org/zcmp>.
+
+- Invoke `cmp` on two files compressed via `gzip`:
+
+`zcmp {{path/to/file1.gz}} {{path/to/file2.gz}}`
+
+- Compare a file to its gzipped version (assuming `.gz` exists already):
+
+`zcmp {{path/to/file}}`