Make anchors in Quick Docs relative to page URL

For #8718.
I'm curious as to why you changed from using `url.charAt(0) !== "/"` to `url.indexOf("/") !== 0` ? `charAt()` is more efficient because it only checks the first char for a slash, where `indexOf()` will keep checking chars until it finds a slash or end-of-string (even though you're only interested in the first char).