Grammar, spelling, and formatting fixes in documentation

[cjlarose]

Besides standard English changes, I did the following:

• Replaced all "smart quotes" (“ and ”) with dumb quotes ("). In places where smart quotes were used in text, they were often wrong (two left-hand quotes or two right-hand quotes). Where they were used in code snippets, they resulted in invalid code. Jekyll converts dumb quotes to smart ones automatically anyway (ignoring code snippets).
• Replaced a bunch of instances of "JSON stringify algorithm" with "JavaScript's toString algorithm". This was already correct on the "Paths" page, but was consistently incorrect on the "JSON Graph" page. I think "JSON stringify algorithm" means JSON.stringify which is a way to turn a JavaScript Array or Object into a JSON string. "JavaScript's toString algorithm" is just calling .toString on something, like (4).toString()