[Feature request] Implement `--appimage-man`

**User story:**

> *"As a user, I want to be able to access and read all man pages (and potentially other documentation) that an AppImage maintainer (or the developer of the respective app) has embedded into the image."*

I'd expect the man page(s) to get printed in the terminal output, when I run
*`myApp.AppImage --appimage-man`*
from the command line. Should there be several relevant man pages, I'd expect to find out a standard way to select the one I need....

But maybe you guys can come up with an even more convenient method?

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^{(This feature request may need thorough discussion and preparation work first. A standard way to implement this should possibly be specified in the [AppImage spec draft](https://github.com/AppImage/AppImageSpec/blob/master/draft.md) document.)}