remove - vars, update docs to generate from data yml files, update mixins, and generate card, button, bg, and color classes from the sass map

* Remove comment that duplicated some code
* Use transition mixin whenever possible
* Create a new function to reduce duplication
* Use the new `breakpoint-infix` method

* Clean up some utilities

- Align CSS properties
- In `_spacing.scss`, we had a comment saying what 'a' was for, but we removed that so this comment no longer applies

* Remove '-xs' from `.navbar-toggleable-xs` and remove duplication

* Fix outdated classes to use newer ones

Websites might use different weights such as medium or thin or use light as default weight instead of normal.
Also referenced: #18605 and #16170.

- Rather than mix multiple properties in our color utilities, this splits all color and all background utils into separate classes.
- Adds new .text-white class to help lighten text color for darker backgrounds

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* `.font-normal` is too generic. Rename it to `.font-weight-normal` for clarity.
* Rename `.font-bold` to `.font-weight-bold` so as to parallel `.font-weight-normal`.
* In docs, gloss "weight" term in relation to fonts for the benefit of non-typographiles.

Refs #18433

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Erratum from 26da610b
Refs #18001

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
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Closes #18314 by merging a tweaked version of it.

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Refs #16836
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- Simplifies variables usage
- Makes components and brand variable usage more consistent (dark bg and white text throughout instead of some mixed light and dark bgs)
- Very likely means lower contrast and thus no more AA compliance (we'll want to fix that eventually obviously)

Fixes #13583: Move .sr-only and .sr-only-focusable to mixins, then use them to create classes in _utilities.scss