This commit allows nested structures for accordions. Also a part of
the documentation about data-children is removed because this
functionality didn't work and it's not applicable anymore.

Tests with the collapse accordion are also a bit adjusted to the new
situation.

Fixes #25127.

The download link for Entypo (on its website) is invalid. It's a Dropbox link which now results in a 403 error. The website was last updated in 2015 (as seen in the footer).

"Choose file ..." text is now written inside the label tag, so the translation is outside the css scope. 
$custom-file-text variable should only contain "Browse" translations.

Closes #25067

- Changes the wrapping label to a div so we can style the label instead of another element while also supporting form validation.
- Fixes form validation styles for custom file input (closes #24831).
- Updates docs with validation styles (also adding example feedback text while I was there) and new how it works section.

* Fix input group border radius issue (#25075)

* Remove migration note

- All buttons In input group do not have to be the same element now.

* lint

Adds some additional context on `.form-group` usage

* Restyle code element, clean up variables

- Removes the padding and background-color
- Changes the color to a brand color ()
- Add new variables to kbd element styles to account for removing the code ones
- Remove overrides that were needed previously

* only break between words, and override it in the pre

* make file inputs 100% wide

* scope custom file changes in input group for sizing

* shorter button labels for responsive

* narrower cards

* button was too wide

* downsize

* fewer links in pagination

* redo cell text

* improve overflow docs

* match layout behaviors

* ditch the indicator as separate element for psuedo-elements on the label

* move disabled to attribute only on input

* redo default inline check to support new markup

* redo inline forms

* clean up vars

* update validation mixin to new structure

* update checks in docs

* linting for for/id attributes

* Rewrite input group component

* Set the feedback to 100% width for input group

* Move from .row to .form-row for tighter layout

* no need for custom feedback here, we're using browser messaging

* add input group to validation examples

* add note about validating multiple

* migration note added