Add the new ROS 2 cheat sheet

[civerachb-cpr]

Information is largely the same as the old Melodic/Noetic PDF in terms of contents; it's intended to be an at-a-glance summary, or the sort of thing you could format as a poster to hang on the wall if you're into that sort of technical signage. Or maybe get it printed upside-down on a t-shirt, like some Linux cheat sheets I've seen.

Resolves RP-3839

[civerachb-cpr]

I think all of the information from the original sheet, with relevant ROS 2 modifications, is present.

We should be able to review this & make any further changes as necessary.

[jhiggins-cpr]

Just noting that these changes will only show up in the "Next" tab, which will become Jazzy. If we also want them for Humble, we'll either need to copy them over to the appropriate directory or create a shared component and instantiate in both locations.

[civerachb-cpr]

All of the content should be pretty distro-agnostic, so once this gets approved it should be pretty trivial to copy it to humble if we decide we want that.

[tonybaltovski]

LGTM with one comment