Use our example shortcode's `class` parameter instead.

* Remove mention of icons from migration since they're not part of the core project

* Cleanup icons page to mention Bootstrap Icons

* Add variables for shades and tints of each major color

* getting-started/theming.md: use a `range`.

Add optional `$color` argument to `button-variant()` and
`button-outline-variant()` for additional flexibility.

* Scope .form-check-input layout to the .form-check parent

* Remove margin-bottom from list-group-items in case they are label elements

* document .form-check in input groups and list groups, and cleanup the no-label examples in the main .form-check docs

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