pngcheck: add page

[ajrequenez]

• The page(s) are in the correct platform directories: linux
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The PR title conforms to the recommended templates.
• **Version of the command being documented (if known):**3.0.3

Although not listed in the pen-testing tools, this is a useful forensics utility used in CTFs.

Related to #6863

[CLAassistant]

All committers have signed the CLA.

[pixelcmtd]

@ajrequenez Your first commit still has the email [email protected] in it, which CLA bot can't check. Can you please use some git magic (I think you need to squash) to fix that?