graphviz filter commands: add pages (#6401)

pages/common/gvcolor.md

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+# gvcolor
+
+> Colorize a ranked digraph with a range of colors.
+> Graphviz filters: `acyclic`, `bcomps`, `comps`, `edgepaint`, `gvcolor`, `gvpack`, `mingle`, `nop`, `sccmap`, `tred`, & `unflatten`.
+> More information: <https://graphviz.org/pdf/gvcolor.1.pdf>.
+
+- Colorize one or more ranked digraph (that were already processed by `dot`):
+
+`gvcolor {{path/to/layout1.gv}} {{path/to/layout2.gv ...}} > {{path/to/output.gv}}`
+
+- Lay out a graph and colorize it, then convert to a PNG image:
+
+`dot {{path/to/input.gv}} | gvcolor | dot -T {{png}} > {{path/to/output.png}}`
+
+- Display help for `gvcolor`:
+
+`gvcolor -?`