pambrighten, ppmbrighten: add pages (#12363)

pages/common/pambrighten.md

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+# pambrighten
+
+> Change a PAM image's saturation and value.
+> More information: <https://netpbm.sourceforge.net/doc/pambrighten.html>.
+
+- Increase the saturation of each pixel by the specified percentage:
+
+`pambrighten -saturation {{value_percent}} {{path/to/image.pam}} > {{path/to/output.pam}}`
+
+- Increase the value (from the HSV color space) of each pixel by the specified percentage:
+
+`pambrighten -value {{value_percent}} {{path/to/image.pam}} > {{path/to/output.pam}}`

pages/common/ppmbrighten.md

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+# ppmbrighten
+
+> This command is superseded by `pambrighten`.
+> More information: <https://netpbm.sourceforge.net/doc/ppmbrighten.html>.
+
+- View documentation for the current command:
+
+`tldr pambrighten`