merged "theory" in the language-agnostic section

Hi,
there was some chapter named "Theory" in the language section.
I deleted it and merged the books in the language-agnostic section.
I did not know where to put "Networks, Crowds, and Markets", so i put it in Misc. And this HoTT also fits in the Math section. I do not know if this woulkd be a better idea.