Stack in cards in one column when < md

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