Add hover anchor links?

[tadhg-ohiggins]

We generate anchor links as part of the content guide already, for example https://pages.18f.gov/content-guide/technical-and-interface-writing/#guidelines, but if you're on that page and you want to acquire that URL, you have to either add it manually or copy it out of the source code.

There is at least one JavaScript library for making this easier for users: https://milanaryal.com/adding-hover-anchor-links-to-header-on-github-pages-using-jekyll/. I'm not necessarily endorsing that particular library, but I think we should add this to the guide (and probably suggest that this kind of thing is a good idea for content generally).