Closes #29630

Co-authored-by: XhmikosR <xhmikosr@gmail.com>

* Simplify disabled styles

* Mention pointer-events usage

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

* v5: Simplify navbars by requiring containers

* Update migration docs for navbar

* Remove mention of icons from migration since they're not part of the core project

* Cleanup icons page to mention Bootstrap Icons

Add optional `$color` argument to `button-variant()` and
`button-outline-variant()` for additional flexibility.

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

From #28948.

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.

* 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...

* Revert "Fix readonly-plain-text with button addon (#25871)"

This reverts commit ff40e003.

* Add migration comment

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

* Typo

Previously deprecated in v4.x, this clears out the now unused Sass option and removes some unused mixins. Arguably we could remove more, but I like the hover-focus mixin and we make extensive use of it across the project.

* Migration docs page for v5

* Add JS section, tweak some copy

* Update migration.md

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

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

* Prep for v4.3.0

* More updates for v4.3.0.

* rerun to build docs source map