Document .card-*-outline variants

Added !important to hover

.container, .container-fluid should be conditional upon $enable-grid-…

README: Add badges for Packagist and Atmosphere

v4: Rename .btn-*-outline to .btn-outline-*

fix justified navbar

v4: Custom forms (disabled state, revamp, etc)

- add more details on custom checkboxes and radios for how we handle the states and how the css works
- create a disabled custom checkboxes and radios example

- Less nesting with more specific classes
- New class names for most elements to avoid unnecessary and potentially confusing shorthand
- Require new class and markup for the description (though it's only necessary for disable states, it's now part of the entire component's markup for all states just in case)
- Change up colors and variables for regular and disabled states

Remove nonstandard `zoom: 1` from media object

Fix margins in highlighted code in docs

Refs #18767
[ci skip]

v4: font-weight: normal on labels

Grunt: Extract lint-docs-css task

[skip sauce]

v4: Docs Sass linting

Docs: accessibility fix-up of collapsible content navbar, change site-wide main navbar

remove `<div class="bd-sidebar">`

Docs: add note and advice about .disabled and keyboard focus

Remove .dl-horizontal; it's redundant to the grid system

Docs: tweaks to content code