pngcheck: add page (#9015)

pages/linux/pngcheck.md

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+# pngcheck
+
+> Forensics tool for validating the integrity of PNG based (`.png`, `.jng`, `.mng`) image files.
+> Can also extract embedded images and text from a file.
+> More information: <http://www.libpng.org/pub/png/apps/pngcheck.html>.
+
+- Verify the integrity of an image file:
+
+`pngcheck {{path/to/file.png}}`
+
+- Check the file with [v]erbose and [c]olorized output:
+
+`pngcheck -vc {{path/to/file.png}}`
+
+- Display contents of [t]ext chunks and [s]earch for PNGs within a specific file:
+
+`pngcheck -ts {{path/to/file.png}}`
+
+- Search for, and e[x]tract embedded PNGs within a specific file:
+
+`pngcheck -x {{path/to/file.png}}`