graphviz filter commands: add pages (#6401)

pages/common/tred.md

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+# tred
+
+> Compute the transitive reduction of directed graphs.
+> Graphviz filters: `acyclic`, `bcomps`, `comps`, `edgepaint`, `gvcolor`, `gvpack`, `mingle`, `nop`, `sccmap`, `tred`, & `unflatten`.
+> More information: <https://www.graphviz.org/pdf/tred.1.pdf>.
+
+- Construct the transitive reduction graph of one or more directed graphs:
+
+`tred {{path/to/input1.gv}} {{path/to/input2.gv ...}} > {{path/to/output.gv}}`
+
+- Display help:
+
+`tred -?`