Adding an image to man7/Audio-subsystem.md referencing a non-icon image
file in the same directory broke the automated build of the manpages
website, which was not prepared to handle this case.
Previously all the added icons weren't available in the online version
of the man pages. This patch adds functionality to copy all the used
icons over when assembling the HTML version.
The special URL links (help://man) and the application opening links now
work on the man page website. While the page links are translated
correctly, the application launch can't be implemented. For this reason,
an explanatory error page is shown instead.
