Add a target to generate the gh-pages files.

It sets `github: true` for us.
Merge pull request #15134 from twbs/docs-gh

Add a target to generate the gh-pages files.
Update devDependencies.
Merge pull request #15183 from twbs/deps

Update devDependencies
Move inline CSS to docs.css.
Carousel example: move the Base64 images outside of the HTML.