[ci skip]

Change `hidden-sm-up` to `hidden-md-up` as the clearfix divs should be visible on both `xs` and `sm` devices (widths of 6 columns).

* 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

https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/8282613/ has been marked as Fixed.
Refs #20335.

[skip sauce]

* Fixes #19796: Remove mention of tooltip auto placement

* add note to migration docs

h/t @bardiharborow for opening https://wpdev.uservoice.com/forums/257854-microsoft-edge-developer/suggestions/15944476
Refs #20143
[skip sauce]

[skip sauce]

* Fix docs for heading sizes (sizes here were from v3)
* Added rem sizes to header size docs
* Fixed erroneous px sizes

* 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

Closes #20507

[skip sauce]

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

Fixes #20484

To be prefixed with `.table-`.