Add explanatory sentences and doc links to all listing pages

[johncowen]

Description

It would be nice to have some sort of introductory sentence or two on certain pages, plus a small "icon button" link to a docs page for that page. I think we used to have something similar to this, but it sometimes stood out more than the content itself, we should think about making it visible but not too "standout" hence the smaller docs button and short intro text rather than large paragraphs.

I think this is mostly "all listing pages" but there might be the odd non-listing page that where we also want to do this. I 'think' there are some pages where we definitely don't want this but not totally sure, but we can decided as this feature is worked on.