[Docs] Clarify suggested use of custom validation styles, add note about resetting form appearance after validation (#25628)

* Expand/clarify recommendation for using custom validation styles

Seems support for correctly announcing browser default navigation bubbles/notifications has improved recently. Now it seems only Chrome (at least on Win and Android, not tested on macOS) is the odd one out.

However, we should really make sure the custom validation messages are also tied correctly to their relevant form controls - need to dig out the PR that will ensure that.

* Add note about resetting form appearance after submission

xref https://github.com/twbs/bootstrap/issues/25626

Closes https://github.com/twbs/bootstrap/issues/25611

* Dropdown - Allow to disable Popper.js style

* Update dropdown.js

* Update dropdown.html

* copy changes

* Move helpful code comments

* Add .carousel-fade option to Sass

* Document example of the fade carousel

* more logical warning of the .active class on carousel items

* Add .dropdown-item-text class

* Add example of plain text within a .dropdown-menu, too

* reorganize, simple/clearer headings

Opera 19 was released in January 2014 and auto-updates so I suggest no need to mention about browser support for this old version.

Removed '**Animated progress bars don't work in Opera 12**—as they don't support CSS3 animations.' note.

Bootstrap 4 doesn't (and doesn't need to) support the ancient Opera 12 released in 2012 with a 0.04% marketshare so no need to mention.

* Make dropright snippet the same as demo

* Add missing .dropdown-toggle-split class

* Make clear the split button label the same as demo

fixes #25288

* Add additional form validation examples

Fixes #24811.

This adds .valid-feedback to our custom styles and server side examples; previously we ommitted this to suggest you don't always need valid feedback. In addition, this adds examples of the .{valid|invalid}-tooltip classes with a new subsection in the Validation docs.

* Update validation tooltip styles to remove fixed width; instead should retain itself to the parent element

* update ids

* finish docs paragraph, mention position: relative

* Remove incorrect roles from accordion, tweak/expand accessibility info
* Use `<button>` elements rather than links for the accordion

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.

"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.

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