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/pnmhisteq.md

@@ -0,0 +1,16 @@
+# pnmhisteq
+
+> Histogram-equalize a PNM image.
+> More information: <https://netpbm.sourceforge.net/doc/pnmhisteq.html>.
+
+- Increase the contrast of a PNM image using histogram equalization:
+
+`pnmhisteq {{path/to/input.pnm}} > {{path/to/output.pnm}}`
+
+- Only modify grey pixels:
+
+`pnmhisteq -grey {{path/to/input.pnm}} > {{path/to/output.pnm}}`
+
+- Do not include black or white pixels in the histogram equalization:
+
+`pnmhisteq -no{{black|white}} {{path/to/input.pnm}} > {{path/to/output.pnm}}`