pambackground, pamstretch-gen, pamstretch, pbmclean, pbmmask, pbmpage, pbmpscale: add pages (#12237)

* pambackground, pamstretch-gen, pamstretch, pbmclean, pbmmask, pbmpage, pbmpscale: add pages

* add missing linefeed

* Update pages/common/pamstretch.md

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

* Apply suggestions from code review

Co-authored-by: Vitor Henrique <87824454+vitorhcl@users.noreply.github.com>

---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: Vitor Henrique <87824454+vitorhcl@users.noreply.github.com>

pages/common/pbmclean.md

@@ -0,0 +1,16 @@
+# pbmclean
+
+> Clean up a PBM image by erasing isolated black and white pixels.
+> More information: <https://netpbm.sourceforge.net/doc/pbmclean.html>.
+
+- Clean up a PBM image by erasing isolated black and white pixels:
+
+`pbmclean {{path/to/image.pbm}} > {{path/to/output.pbm}}`
+
+- Clean up only black/white pixels:
+
+`pbmclean -{{black|white}} {{path/to/image.pbm}} > {{path/to/output.pbm}}`
+
+- Specify the minimum number of neighbouring pixels of the same color in order for a pixel not to be considered isolated:
+
+`pbmclean -minneighbours {{3}} {{path/to/image.pbm}} > {{path/to/output.pbm}}`