Add User-oriented search it intro

On looking at the SearchKit docs it all just felt like developer stuff - there was nowhere to explain how to do a join / group by / basic concepts. I think the recipes are great - but there needs to be a way in & somewhere to point people to when they can't figure out their where vs their join

I moved the stuff that is really for site admins & developer down the page - to move the focus to 'how do I use search kit' rather than 'how does it fit in the ecosystem'