Bring back the `/docs` directory

This moves all documentation files to a `/docs` subdirectory to help clean up the file tree. Here are some thoughts on what's changed and why it matters.

* All `.html` files, `/examples`, and Jekyll directories have been moved to a subfolder, `/docs`.
* **By far the biggest win is organization.** It cleans up the file tree for beginners who have no idea what some of this stuff is and makes it easier for us as maintainers (@twbs/team) to stare at this stuff all day.
* **By far the biggest downside is that this duplicates some code.** However, I've tried to keep that to a minimum by only bringing over the minified CSS and JS files. All the fonts are duped though.
* As part of this, the `_config.yml` file has been cleaned up. Nested configs look way better. I was also able to remove all the `exclude` lists since the stuff we really need to compile the docs is now all in one spot, free of extra noise.
* I've also replaced all instances of `{{ page.base_url }}` with `{{ site.baseurl }}`. Better to use an official config setting than a custom one on each page. This does, however, mean that folks who fork Bootstrap need to change the `baseurl` option in Jekyll if they wish to host our docs themselves. I'm not worried in the slightest about this.

/cc @fat, beacuse <3.