* 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

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

- Add inverse examples for all variants
- Use rgba() colors for hover and accent states for easy usage in inverse styles

- Change some things around, add some copy
- Include table variant styles for the inverse tables thanks to bg and color utils

[skip sauce]

Closes #19339
Refs #19102, #19443, #19665
[skip sauce]

Replace the leftover instances with .m-x-auto (and .d-block when necessary)
Closes #19665
Refs #19102
[skip sauce]

- Add/replace callout for color and AT with include

[skip sauce]

Fixup for 27ba4beb
/fyi @mdo
[skip sauce]

- Using the grid classes, they still need clearfixes since rows aren't injected between them all
- Update docs to fix broken classes

[skip sauce]

These are available in _type.scss but not documented.  These classes are useful for styling without adding semantics.

Refs #18458
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It has been removed from the HTML specification: https://github.com/whatwg/html/issues/336
Only Presto Opera ever implemented a special UI for it.

[skip sauce]

also, stray "blocks" in the "Sample output", which doesn't seem to
belong there.

use `<code>` (as it's still semantically code that we're marking up) and
make the example actually multiple lines...

[skip sauce]

Previously, this read "Bootstrap doesn’t apply this automatically as it causes complications to other image formats." 

In this sentence, the number of the subject and the verb don't agree, and the word "this" is unclear.

These can be replaced by their `.text-xs-*` parallels.
This also avoids any complications from interactions between the responsive and non-responsive classes.
(e.g. `<div class="text-left text-md-right">`)

Refs #18300
[skip sauce]

[ci skip]