ENH Styling for api links contains backticks

[emteknetnz]

Issue https://github.com/silverstripeltd/product-issues/issues/627

Ideally would have been done in ss-docs.scss, though I ran into multiple issues when running yarn build

Changes the underline color to grey to match

[GuySartorelli]

This can happen after the beta I think - it won't materially change anything and we really need to get this out.

In any case as discussed this solution won't work - we need to instead recolour a code (i.e. code blocks inside links) to be more similar to the underline provided by the link itself.

We should also do this in scss - if you're having a problem building it we should fix that. There's no rush on this, it's a subjective visual change with no direct material bearing on the actual documentation itself.

While we're doing this it might be nice to make code a (a link inside a <code> tag) look the same as a code (a <code> tag inside a link)