Import cheat sheets from other tools

[amanger-qual]

I love that we can query tldr and cheatsh, but every time we query them, it takes a while to load.

I'd love to be able to have them imported so I can search for them using navi.

As an alternative, I've considered re-writing the entire cheatsheet then uploading myself, but it would be much easier if navi could just import it.

[welcome]

Thanks for opening your first issue here! In case you're facing a bug, please update navi to the latest version first. Maybe the bug is already solved! :)