Add local server instructions for offline access

[SuperCowProducts]

Added instructions to run a local offline copy of the website.

[codeguru42]

For static HTML pages, this shouldn't be necessary. But after I build the book and open index.html directly in my browser, chapter and section links don't work the same as the online version of the book.

The behavior I see is caused by the format of the URLs in the table of contents links. They look like file://home/username/tpil/book/_out/html-multi/Inductive-Types/#defining-the-natural-numbers. If they instead include index.html in the URL, then they will work without running a server: ``file://home/username/tpil/book/_out/html-multi/Inductive-Types/index.html#defining-the-natural-numbers`.

I'm uncertain if this can be fixed directly in the book repo here or if an upstream change in Verso is needed.