* descriptions for getting started pages

* descriptions for layout

* add content page descriptions

* more descriptions, updates to some existing ones

* correct site url

* add social stuff to config for twitter cards

* add twitter meta tags; use large image for homepage and regular card for all others

* add the assets

* more site config

* more social shiz to partial, remove existing meta for the partial, remove page title from homepage for simpler if statements

* Fixes #19796: Remove mention of tooltip auto placement

* add note to migration docs

* Add .img-fluid to card images in columns to ensure proper resizing
* change that variable from regular value to variable
* use calc to figure out a tighter border for card images

- Update docs examples to use newer .form-check suite of classes
- Update form validation mixin to use newer classes as well

Fixes #20484

Remember, there may be children/students using these examples...

remove current HTML-based symbol, add any missing aria-* attributes

Clarify valid values of Carousel's `pause` option

Fixes #18205

[skip sauce]

Fixes #17193: Remove mention and link to non-existant transitions plugin; that intro paragraph was two dupe sentences

Added an example tooltip that uses HTML content, and expanded the
description of the html option to clarify usage.

Since the "remote" option is no longer available in v4 the reference to the "loaded.bs.modal" event is no longer needed, too.

- Really we didn't need to do any of that strip units stuff to generate six new variables, two for each button size
- Using sibling selectors, we can target those split button dropdown toggles using the .dropdown-toggle-split class, and adjust padding and margin as needed
- Now, we nuke the margin-left from the ::after generated caret and tighten up the padding so that those split toggles don't look huge next to their main button

- Includes new class
- New docs to support it
- Rearranged older docs

- Add lighter gray text to disabled state for .form-check
- Separate radios and checkboxes example
- Clarify docs

- Move disabled radio and checkbox styles to Reboot
- Collapse .radio and .checkbox into single class, .form-check
- Collapse .radio-inline and .checkbox-inline into single class, .form-check-inline
- Require classes for sub-elements in both new classes

[ci skip]

Fixes #18573: Change name of .form-control-label to .col-form-label (and .form-control-legend to .col-form-legend) to reiterate use for only grid layouts