Remove checkbox/radio toggle from button plugin in favor of a CSS only solution

Created by: MartijnCuppens

Alternative solution for #28463. This PR also separates the checkboxes docs from the button groups and buttons.

• The Button plugin is removed in favor of a CSS only solution, a checkbox/radio with a .btn-toggle class. This solution works with as well normal as outlined buttons.
• The checkbox and radio toggles docs are moved to the form checks docs.
• An active state example for links is added to the button groups
• Some outline examples are added to the button groups.

This PR removes the button toggle functionality (toggling active classes to buttons). I think it's better to just rely on checkboxes to toggle states, since the the js solution introduces issues like https://github.com/twbs/bootstrap/issues/25122.

Note the increase of CSS file size will be negligible once we use custom properties for our button component.

• Toggle buttons removed from https://deploy-preview-30650--twbs-bootstrap.netlify.app/docs/4.3/components/buttons/, the .active documentation is kept.
• Active link & oulined example added to https://deploy-preview-30650--twbs-bootstrap.netlify.app/docs/4.3/components/button-group/
• Button toggles documentation added to https://deploy-preview-30650--twbs-bootstrap.netlify.app/docs/4.3/forms/checks/#toggle-buttons
• Migration docs added to https://deploy-preview-30650--twbs-bootstrap.netlify.app/docs/4.3/migration/#buttons

Checks:

• Check for .btn-group-toggle leftovers in docs.
• Check for data-toggle="button" leftovers in docs.

Closes #28463 Closes https://github.com/twbs/bootstrap/issues/30615 Closes https://github.com/twbs/bootstrap/issues/25122 since the button plugin is removed