pnmhisteq, pnmhistmap, pnmindex, pnminvert: add pages (#12189)

* pnmhisteq, pnmhistmap, pnmindex, pnminvert: add pages

* Update pages/common/pnmindex.md

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/common/pnmhistmap.md

@@ -0,0 +1,16 @@
+# pnmhistmap
+
+> Draw a histogram of a PNM image.
+> More information: <https://netpbm.sourceforge.net/doc/pnmhistmap.html>.
+
+- Draw a histogram of a PNM image:
+
+`pnmhistmap {{path/to/input.pnm}} > {{path/to/output.pnm}}`
+
+- Draw the histogram as dots instead of bars:
+
+`pnmhistmap -dots {{path/to/input.pnm}} > {{path/to/output.pnm}}`
+
+- Specify the range of intensity values to include:
+
+`pnmhistmap -lval {{minval}} -rval {{maxval}} {{path/to/input.pnm}} > {{path/to/output.pnm}}`