Promote the "upgrading" guide

[WasabiFan]

It shows up low on the tutorials page and we rarely mention it, so I doubt that many people know it exists.

[ddemidov]

Include the link into new kernel/release announcements?

[WasabiFan]

That makes sense; I'll try to open a PR at some point to add that to existing releases (or at least, the most recent). Also, we should link to it somewhere on the "downloads" page.

I've been thinking it would make sense to have a page for an ev3dev "cheat sheet" with links to the various pages describing essential operations. Things like shutting down, upgrading, deploying code, etc. could use a good pathway from the initial guide.