Merge the "overview" and "concepts overview" pages

!179 (merged) had added a lot of docs on workers, processes and models. We were still using MkDocs, and each section header had to be its own page, so we had tiny pages with one or two sentences just so that the sections weren't completely empty. With Antora, those pages have been moved to Overview pages, and the sections are no longer clickable links, they are just dropdowns.

This means there now are Overview and Concepts overview pages, one after the other, under the Workers, Processes and Models sections. We can merge them.