* create responsive containers

provide more flexibility and allow the user to determine when containers switch from fluid to fixed width.

* fix the base container code

this commit fixes the non-media portion of the generated CSS. I learned about the `@extends` directive and was able to put it to good use.

I create a new temporary map that contains all the main `$container-max-widths` and join it to our 2 special cases of 'xs' and 'fluid'.  Then we loop through that and, with the appropriate infixes, extend our placeholder

* formatting for style

forgot to run my tests before the last push, i think these are better.

* finish incomplete comment

* fix the responsive containers

using the `@extend` directive I was able to clean up this code

* fix responsive containers in the navbar

mostly we just look through all of our breakpoints so we can include all of the responsive container classes in the tweaks we have to do for the navbar (redec...

* Use escape-svg() function

* Update theming.md

This was causing issues for RTL, see https://github.com/twbs/bootstrap/issues/28238

While #28450 overhauled a ton of forms work, the  support for this custom file input was ineffective. This updates the selector to target the custom field and gray out the entire field.

Fixes #28176.

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.

Corrected minor spelling error.

* 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

x-ref #28987

fixes issue #28892

Co-Authored-By: Shohei Yoshida <fellows3@gmail.com>

Co-Authored-By: Shohei Yoshida <fellows3@gmail.com>

* Remove unused classes/styles and use utilities instead of specific styles
* Use `getElementById` instead of `querySelector`