Make CTA boxes more obviously clickable, especially on mobile.

[hypha]

          On mobile, there's no real indication that you can click on these three cards. With a mouse, you get a pointing hand cursor on hoover, but there's obviously no such indication on mobile.

Originally posted by @SebastiaanZ in #235 (comment)

[egeakman]

@hypha Is this still an issue?

[hypha]

I think this has not changed from the time the issue was raised (by @Vaibhavs10 and @SebastiaanZ) in the original comment. So I imagine it is still an issue. But I also think it might be a good idea to ask people who are experienced in accessibility to weigh in (@ADubhlaoich comes to mind).

[ADubhlaoich]

In the wider scope of UX interactions this feels closely related to #697, too.

I will review the current state of the website for recommendations (And maybe open a PR myself, since things are quiet now post-conference).