pbmnoise, pgmhist, pgmkernel, pgmnoise, pgmramp, pgmtexture: add pages; ppmhist: modify page (#12202)

pages/common/pgmramp.md

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+# pgmramp
+
+> Generate a greyscale map.
+> More information: <https://netpbm.sourceforge.net/doc/pgmramp.html>.
+
+- Generate a left-to-right greyscale map:
+
+`pgmtexture -lr > {{path/to/output.pgm}}`
+
+- Generate a top-to-bottom greyscale map:
+
+`pgmtexture -tb > {{path/to/output.pgm}}`
+
+- Generate a rectangular greyscale map:
+
+`pgmtexture -rectangle > {{path/to/output.pgm}}`
+
+- Generate a elliptical greyscale map:
+
+`pgmtexture -ellipse {{path/to/image.pgm}} > {{path/to/output.pgm}}`
+
+- Generate a greyscale map from the top-left corner to the bottom-right corner:
+
+`pgmtexture -diagonal {{path/to/image.pgm}} > {{path/to/output.pgm}}`