[CSS] support for system-ui font family

[Rumyra]

Add support for the new 'system-ui' generic font family Lots of discussion in this bug but it does appear this is now supported

Acceptance Criteria

• The listed features are documented sufficiently on MDN
• BCD is updated
• Interactive example and data repos are updated if appropriate
• The content has been reviewed as needed

For folks helping with Firefox related documentation

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes if feature is enabled in release
  or
• Add entry to Firefox experimental features page if feature is not yet enabled in release

Features to document

• Would imagine this is documented, so update to bcd and rel notes here

Related Gecko bugs

• Add support for the new 'system-ui' generic font family