File input belongs to this page in doc, added variables in last.

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

* WIP: Mention variables, mixins, and loops in docs

* Add Sass sections to component pages

* add sass docs for forms and content

* Update buttons.md

* Remove empty mixins sections

* Massive update to utilities and some consistency changes

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

Shorter URLS which are faster as they avoid HTTP redirects

* Add disabled state example to the .form-control page

* Document disabled attribute on select too

* Add disabled example for file input

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

* Rename `sr-only`/`sr-only-focusable`

To be more representative of the fact that these are not necessarily "screen reader" specific, but actually apply to assistive technologies in general (and also things like Alexa/Siri/etc). Goes hand-in-hand with #31133

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

Also add v4.5.0 in versions and keep README.md pointing to v4.5.0 so that there are no broken stuff.

- in particular, controls that just relied on `placeholder`, as this is still not officially/consistently supported as a valid way to provide an accessible name (though some browsers do use it as a fallback)

Co-authored-by: Mark Otto <markd.otto@gmail.com>

- Add example to the new form control docs
- Reset the [list] selector in Reboot to hide the random dropdown arrow in Chrome

Fix Readonly plain text form alignment

* 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...