Merge website, doxygen pages, and wiki?

Split from #3194 (closed)

Most of our user-facing content is prose inside the doxygen pages, leaving just a few pages on the main website. And the two have completely different styles and layout. We could consider merging in one direction or another: we could remove all the non-API content from doxygen control and move it to the Pages and embed the API content in an iframe (maybe remove the navbar, etc. there); or we could go the other way and try to put all the top-level pages inside doxygen auto-generation.

We could also consider removing the wiki and moving its content into the website, to have everything integrated in one place.