Improve website organization

[mtfishman]

It seems like we can merge "The ITensor Book", "Tutorials", and "Code Formulas" together (maybe all into "The ITensor Book"?).

They all have useful information, but it is too spread out right now. I often spend time looking for a tutorial/sample code I know is there but forgot which link it was put under.

[emstoudenmire]

Hi Matt, I like this idea. I had thought about some of those tutorial topics as not being about ITensor per se (such as how to make an MPO or something like that). But we could always put it as a book chapter anyway, and then have some examples using ITensor code, or not. It’s not that all the book chapters have to be introductory, and some can be rather stand-alone type chapters.