Now there's only one command needed to run the docs: `npm run docs-serve`.

Also, simplify the npm scripts.

See https://github.com/twbs/bootstrap/pull/29368#issuecomment-528725686

- Simplify display headings example markup
- Simplify footer styles
- Remove z-indices hacks in the docs
- Remove redundant modal styling in docs
- Use `escape-svg()` to escape docs sidebar chevron
- Simplify sidebar chevron code
- Cleanup masthead css & fix column width between `md` & `lg`
- Easier to download logo's with the `download` attribute.
- Changed some color codes into variables
- Cleanup brands css

This broke IE11.

* remove load event
* use `forEach` and `querySelectorAll`
* simplify check

It always used the first placeholder's class before.

- Rename and move the variable to variables file
- Move code to the grid file
- Use the mixin to generate our own classes
- Wrap in a grid classes enabled conditional
- Document the mixin

Trying to find a new way to do responsive card decks while not locking ourselves into the cards themselves. My thinking here is we can easily control the column (.col) width by the parent, but I don't know how many we need (have 0-5 now) across each breakpoint. This works for cards so far, and I think could get us equal height, too.

The example shortcode already adds this.

Use the built-in utilities instead.

Also, add a warning in _root.scss so that we don't miss it again.