- Enable RFS
- Add documentation
- Disable doc font size overrides

* Add variables for shades and tints of each major color

* getting-started/theming.md: use a `range`.

Now there's only one command needed to run the docs: `npm run docs-serve`.

Also, simplify the npm scripts.

Use the built-in utilities instead.

Also, add a warning in _root.scss so that we don't miss it again.

While important in the past, it has become obsolete, so removing it seems the best solution.

Drop `color()`, `theme-color()` & `gray()` functions  in favor of variables. The functions just called a `map-get()` of a map where just the variables were defined. 

Also the `theme-color-level()` now accepts any color you want instead of only `$theme-colors` colors. The first value now is a variable instead of the `$theme-colors` key.

* Use escape-svg() function

* Update theming.md

Media queries do not support CSS custom properties (CSS variables), so we've had some useless variables lying around since shipping v4. This finally removes them and the associated documentation content that explained the situation.

Fixes #29012.

* Initial spike of consolidated form checks

* Stub out forms rearrangement

- Prepping to drop non-custom file and range inputs
- Prepping to merge custom and native checks and radios (with switches)
- Prepping to merge custom select with form select
- Moving docs arround so forms has it's own area given volume of CSS

* Move input group Sass file to forms subdir

* Start to split and move the docs around

* Simpler imports

* Copyediting

* delete overview file

* Remove commented out code

* remove the custom-forms import

* rewrite flex-check as form-check, replace all custom properties

* Remove old forms doc

* stub out new subpage link section

* update migration guide

* Update nav, forms overview in page nav, and descriptions

* fix check bg position

* fix margin-top calculation

* rename .custom-select to .form-select

* Update validation styles for new checks

* add some vertical margin, fix inline checks

* fix docs examples...

* Prevent .sr-only-focusable overriding properties when being focused

* Typo

I think it makes more sense to link to the Bootstrap package on Yarn here (like the package link for NPM above)