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

Merge branch 'docs-border-utilities' of https://github.com/ysds/bootstrap into ysds-docs-border-utilities

The append and prepend classes vertically aligned items in the center
For textareas this meant the addon would aling vertically instead of stretch and have it's contents vertically aligned
these changes fix that so everything is aligned how it should be

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.

* Base the outline button :active color on the background, fixing a contrast issue

* Only apply focus outline to active when it's focused

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.

Position: sticky is included in Edge 16

https://crbug.com/781344

Closes #23319. Both `.text-muted` and `.btn-outline-secondary` (and indeed all secondary items) make use of `$gray-600`. New value provides a contrast ratio of 4.69.