Add an example tour using TourGuide JS

[llewelld]

Adds an example tour using TourGuide JS, to test out its capabilities. The content and coding is just for testing: in no way final!

For TourGuide JS, see: https://tourguidejs.com/

For comparison with #564.

Initially I thought otherwise, but having tried it it's now clear that TourGuide can also be used without having to make any significant changes to the original HTML (it just needs appropriate ids added so the elements can be refered to).

Styling aside the main difference I notice with Shepherd.js is that TourGuide prevents interaction with anything on the page. In contrast, Shepherd.js allows the user to interact with the highlighted element.

Not allowing the user to interact with anything makes it easier to create the tour (no need to worry about users messing with things mid-tour), but arguably isn't as nice for the user.

Shepherd.js makes it easier to dynamically change the labels on the buttons, but I'm not sure that's going to be too important.

Unfortunately, like Shepherd.js, TourGuide also has trouble with a phone-sized display, so we'll have to figure out a solution to that in either case (e.g. disabling the tour on small-screen devices as you suggested earlier @gedankenstuecke).

Contributes to #535.