* Clean up some utilities

- Align CSS properties
- In `_spacing.scss`, we had a comment saying what 'a' was for, but we removed that so this comment no longer applies

* Remove '-xs' from `.navbar-toggleable-xs` and remove duplication

* Fix outdated classes to use newer ones

* Update clearfix to use block instead of table display (also reorder properties for linting)
* update docs snippet for clearfix mixin—was apparently still using Less syntax and had old clearfix hack (even before the block change in this PR)

* Clean up _flex.scss a little

This commit just cleans up the formating of _flex.scss by changing
it so that it uses a single `@include media-breakpoint-up` instead
of multiple. It also aligns all of the CSS properties so it looks
a bit nicer.

* Remove `-xs` from flex classes

* fix form-inline with flex enabled
* grunt
* fix alignment of labels
* shorter if syntax
* add new form example to docs for now
* update inline form docs usage guidelines
* responsive margins
* better margin utils
* fix sizing of .form-check
* flexbox alignment of .form-check
* no need to change direction
* support custom controls in inline form, for default and flex modes
* add example of custom select and checks to docs
* remove hidden and visible label variants since we cover that in the usage guidelines at the start and include hidden labels everywhere
* use property value instead of layout name
* apply to all labels
* add a visible label, space it out
* add id

* Add rounded-0 class

* grunt

* add border utilities for removing borders

* clean up comments

* add basic border docs

* docs styles for border utils

Fixes #19107.

grid-breakpoint for sm is 576px
https://github.com/twbs/bootstrap/blob/v4-dev/scss/_variables.scss#L186

current document shows 544px, but it should be 576px

* Add some flexbox nav components
  - Includes .nav-justified for inline, tab, and pill nav components
  - Includes example of using the flex utils (.d- and .flex-items-) for centered nav
* redo heading hierarchy a bit

Refs https://bugs.chromium.org/p/chromium/issues/detail?id=510079
Closes #21120

[skip sauce]