More docs at your finger tips

[behrmann]

This adds the feature to pass an argument to the documentation command, so that the man page we've grown in addition can be viewed easily as well.

[keszybz]

Looks reasonable…

[DaanDeMeyer]

We discussed this in chat but I'd prefer mkosi-initrd learns a --documentation switch to show its documentation.

Doing the same for mkosi-sandbox is harder without introducing dependencies on the rest of mkosi but it could be done if show_docs() is made independent of the rest of mkosi and is implemented using only stdlib stuff.

[behrmann]

I'll rework it next week. I think it's fine to do this both for mkosi-initrd and mkosi-sandbox by lazily using the show_docs stuff, which in the case of mkosi-sandbox will just not work within the sandbox, but I think that's fine, people will usually have another terminal open.