spottopgm, rlatopam, rgb3toppm, ppmtorgb3: add pages (#15354)

* fix ci

* spottopgm, rlatopam, rgb3toppm, ppmtorgb3: add pages

* fix typo

pages/common/spottopgm.md

@@ -0,0 +1,16 @@
+# spottopgm
+
+> Convert a SPOT satellite image to PGM format.
+> More information: <https://netpbm.sourceforge.net/doc/spottopgm.html>.
+
+- Convert the specified SPOT image to PGM format:
+
+`spottopgm {{path/to/file.spot}} > {{path/to/output.pgm}}`
+
+- Extract the specified color channel:
+
+`spottopgm -{{1|2|3}} {{path/to/file.spot}} > {{path/to/output.pgm}}`
+
+- Extract the specified rectangle from the input image:
+
+`spottopgm {{first_col first_row last_col last_row}} {{path/to/file.spot}} > {{path/to/output.pgm}}`