Add code structure diagrams to Wiki

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This is just a notification that I've added a wiki page with a number of diagrams detailing the organizational structure of the code. They only cover the main branch, but I can keep them updated as new branches are merged into it.

These are intended to be broad overviews of objects and tasks, perhaps useful to new developers starting to work with the code.

(Sorry about having to download PDFs. Github overrides the HTML to force them to be downloaded rather than viewed. I might wrestle them into another format at some point.)