Improve the video demonstration

[zaeph]

a337496 added a commented video demonstration to README.md to make it easier to demonstrate some of the features of ORB.

I've done it late at night yesterday with little to no preparation, and it shows, but it's more of a proof of concept than the final version.

Some notes on the recording:

• I haven't made it clear what ORB actually does: I should have laid more emphasis on the template expansion we provide on top of Org-roam's, which is prob ably our main feature.
• It's more of a demonstration on how to do research in Emacs/Org-mode than it is a demonstration of ORB.
• It's long: I should aim for < 5 min to keep it short and sweet.
• A lot of rambling, which muddles up the argument.
• Presentation at the start was a good idea, but should have been carried through the entire demonstration.
• Even if I comment as I do, it wouldn't hurt to have a side-window opened with key-bindings and the commands which they run.
• Haphazard demonstration of Org-roam which would just be confusing for people with no knowledge of it.
• Rhythm was rushed, and the diction was inarticulate at times. Also, some English mistakes ('inspred from', a weird proposition-ranging 'else' at the end of a sentence that sounds odd, etc.)

At least, it's better than just the GIF, which I've kept on the page for the sake of having something animated without having to leave GitHub.