Button nested inside of link

[BNewing]

At the moment, there are a few instances of <button> elements being inside of an <a> tag. For example, the 'Learn how you can help' on the home page:

As this links to another section further down the page, semantically it could do with being just a link. The button styles could be moved to the link, and the appearance preserved.

A benefit of doing this would be fixing slightly unusual keyboard focus behaviour. Because there are two interactive elements - both the 'and the`, you have to tab twice to get past the element. It would also be a better experience for screen reader users.

I'm new to this project and not yet in the slack, but I'd be happy to put together a PR if it's welcome! :)