ppmdraw, ppmglobe, pnmmercator: add pages (#15357)

* fix ci

* ppmdraw, ppmglobe, pnmmercator: add pages

* Apply suggestions from code review

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

---------

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

pages/common/pnmmercator.md

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+# pnmmercator
+
+> Perform Mercator transformations on Netpbm images.
+> See also: `pnmglobe`.
+> More information: <https://netpbm.sourceforge.net/doc/pnmmercator.html>.
+
+- Convert a rectangular projection worldmap to Mercator projection:
+
+`pnmmercator {{path/to/image.pnm}} > {{path/to/output.pnm}}`
+
+- Convert a Mercator projection worldmap to rectangular projection:
+
+`pnmmercator -inverse {{path/to/image.pnm}} > {{path/to/output.pnm}}`

pages/common/ppmdraw.md

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+# ppmdraw
+
+> Draw lines, text and more on a PPM image by executing a script.
+> Documentation on the utilized scripting language can be found by following the link below.
+> More information: <https://netpbm.sourceforge.net/doc/ppmdraw.html>.
+
+- Draw on the specified PPM image by executing the supplied script:
+
+`ppmdraw -script '{{setpos 50 50; text_here "hello!"; }}' {{path/to/image.pnm}} > {{path/to/output.pnm}}`
+
+- Draw on the specified PPM image by executing the script in the specified file:
+
+`ppmdraw -scriptfile {{path/to/script}} {{path/to/image.pnm}} > {{path/to/output.pnm}}`

pages/common/ppmglobe.md

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+# ppmglobe
+
+> Generate strips of an image suitable to be glued onto a sphere.
+> See also: `pnmmercator`.
+> More information: <https://netpbm.sourceforge.net/doc/ppmglobe.html>.
+
+- Transform an image to strips that can be cut out and glues onto a sphere:
+
+`ppmglobe {{number_of_strips}} {{path/to/image.ppm}} > {{path/to/output.ppm}}`
+
+- Use the specified color for the areas between the strips:
+
+`ppmglobe -background {{red}} {{number_of_strips}} {{path/to/image.ppm}} > {{path/to/output.ppm}}`