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)

This reverts commit 089e3334.

This reverts commit 1da5b9f7.

This allows for the build to fail if a reference isn't found.

This happens because we have the section kind enabled (needed for examples).

No more Ruby dependency and most important more than 10 times less build time (at least on Windows)!