[ new ] Chapter on details of QTT

[kiana-S]

A while ago (about a month ago at this point, oops) I offered to contribute a section in this tutorial explaining Quantitative Type Theory in depth. I've finally gotten around to typing up a draft of it, so here it is!

A few things to note:

• I didn't see any particularly obvious place to put this section in the table of contents, so I just left it unlinked.
• The new file src/Tutorial/QTT.md will fail to typecheck as a literate Idris file, so I didn't put it in the ipkg module list.

Edits to address any of these issues would be great!