pbmnoise, pgmhist, pgmkernel, pgmnoise, pgmramp, pgmtexture: add pages; ppmhist: modify page (#12202)

pages/common/pgmhist.md

@@ -0,0 +1,25 @@
+# pgmhist
+
+> Print a histogram of the values present in a PGM image.
+> See also: `ppmhist`.
+> More information: <https://netpbm.sourceforge.net/doc/pgmhist.html>.
+
+- Display the histogram for human reading:
+
+`pgmhist {{path/to/image.pgm}}`
+
+- Display the median grey value:
+
+`pgmhist -median {{path/to/image.pgm}}`
+
+- Display four quartile grey value:
+
+`pgmhist -quartile {{path/to/image.pgm}}`
+
+- Report the existence of invalid grey values:
+
+`pgmhist -forensic {{path/to/image.pgm}}`
+
+- Display machine-readable output:
+
+`pgmhist -machine {{path/to/image.pgm}}`