graphviz filter commands: add pages (#6401)

pages/common/gvpack.md

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+# gvpack
+
+> Combine several graph layouts (that already have layout information).
+> Graphviz filters: `acyclic`, `bcomps`, `comps`, `edgepaint`, `gvcolor`, `gvpack`, `mingle`, `nop`, `sccmap`, `tred`, & `unflatten`.
+> More information: <https://graphviz.org/pdf/gvpack.1.pdf>.
+
+- Combine several graph layouts (that already have layout information):
+
+`gvpack {{path/to/layout1.gv}} {{path/to/layout2.gv ...}} > {{path/to/output.gv}}`
+
+- Combine several graph layouts at the graph level, keeping graphs separate:
+
+`gvpack -g {{path/to/layout1.gv}} {{path/to/layout2.gv ...}} > {{path/to/output.gv}}`
+
+- Combine several graph layouts at the node level, ignoring clusters:
+
+`gvpack -n {{path/to/layout1.gv}} {{path/to/layout2.gv ...}} > {{path/to/output.gv}}`
+
+- Combine several graph layouts without packing:
+
+`gvpack -u {{path/to/layout1.gv}} {{path/to/layout2.gv ...}} > {{path/to/output.gv}}`
+
+- Display help for `gvpack`:
+
+`gvpack -?`