Fix documentation that can seem empty

[ivanahuckova]

When visiting Yugabyte) it wasn’t apparent right away that there were docs sub-pages for config and querying. This made me think the docs were basically empty, especially since data source docs have existed on one long page in the past.
There are some other docs pages that have a short “overview” page and put details into sub-page. They use the {{< section >}} tag to display a “table of contents” for their sub-pages inline, so that users who land on the overview page know they should nav to sub-pages. We should implement that.