pnmquantall, ppmquantall: add pages; pnmquant: edit page (#12407)

pages/common/pnmquantall.md

@@ -0,0 +1,13 @@
+# pnmquantall
+
+> Run `pnmquant` on multiple files at once such that they share a common colormap.
+> See also: `pnmquant`.
+> More information: <https://netpbm.sourceforge.net/doc/pnmquantall.html>.
+
+- Run `pnmquant` on multiple files with the specified parameters, overwriting the original files:
+
+`pnmquantall {{n_colors}} {{path/to/input1.pnm path/to/input2.pnm ...}}`
+
+- Save the quantised images to files named the same as the input files, but with the specified extension appended:
+
+`pnmquantall -ext {{extension}} {{n_colors}} {{path/to/input1.pnm path/to/input2.pnm ...}}`