Checkboxes (DB UX Design System v3)

[mfranzke]

The following changes will be part of the Guidelines 3.0 to the checkbox element:

General

• clarify on dark background usage / examples still to keep in the generated examples

Sizes

• Added this whole possibility to define sizes

States

• Added this whole possibility to style the required state

Additionally

• CHANGELOG.md entries, both for prereleases (underneath Unreleased at the beginning) as well as in a hidden (by comment) summary at the beginning
• docs/migrationGuide.adoc
• Test-URLs: .pa11yci and tests/backstop.json

[mfranzke]

Closed in favour of https://github.com/db-ui/mono/issues/362