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.

By moving the Jekyll folders, our examples, docs assets, HTML files, and docs license to the subdirectory, we effectively lose 13 items from the root directory. The root file tree looks like this now:

├─ dist/
├─ docs/
├─ fonts/
├─ js/
├─ less/
├─ test-infra/
├─ .csscomb.json
├─ .csslintrc
├─ .editorconfig
├─ .gitattributes
├─ .gitignore
├─ .travis.yml
├─ CNAME
├─ CONTRIBUTING.md
├─ Gruntfile.js
├─ LICENSE
├─ README.md
├─ _config.yml
├─ bower.json
├─ composer.json
└─ package.json

It's a bit better, but could still use some cleanup. I'm curious if we could punt any of those other files somewhere else. It'd be great if a repo could just have a .files/ directory for this stuff.

/cc @fat, because <3.