* 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

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

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

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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

Refs #19665
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