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/pnmindex.md

@@ -0,0 +1,21 @@
+# pnmindex
+
+> Build a visual index of multiple PNM images.
+> See also: `pamundice`.
+> More information: <https://netpbm.sourceforge.net/doc/pnmindex.html>.
+
+- Produce an image containing thumbnails of the specified images in a grid:
+
+`pnmindex {{path/to/input1.pnm path/to/input2.pnm ...}} > {{path/to/output.pnm}}`
+
+- Specify the size of the (quadratic) thumbnails:
+
+`pnmindex -size {{50}} {{path/to/input1.pnm path/to/input2.pnm ...}} > {{path/to/output.pnm}}`
+
+- Specify the number of thumbnails per row:
+
+`pnmindex -across {{10}} {{path/to/input1.pnm path/to/input2.pnm ...}} > {{path/to/output.pnm}}`
+
+- Specify the maximum number of colors in the output:
+
+`pnmindex -colors {{512}} {{path/to/input1.pnm path/to/input2.pnm ...}} > {{path/to/output.pnm}}`