Make docs use site.url as base instead of static top-level domain, reference to both #7758 and #7767.

This patch works for both following setting in `_config.yml`
* `url + baseurl = /`
  * http://127.0.0.1:9001/
  * http://getbootstrap.dev:9001/ (if I hard code `127.0.0.1 getbootstrap.dev` into hosts file)
* `url + baseurl = /bootstrap/`
  * http://127.0.0.1:9001/bootstrap/
  * http://getbootstrap.dev:9001/bootstrap/ (if I hard code `127.0.0.1 getbootstrap.dev` into hosts file)
  * http://username.github.io/bootstrap/ (github project page for review without local jekyll)

Which means no more hard coding to specific domain name nor folder structure setup in docs's HTML, marks both local jekyll, github project page, and final document site URL with http://getbootstrap.com/ friendly, too ;-)