Release 0.3.0

Added

• Extended Markdown with details blocks
• Extracted extended Markdown functionality into library
• Added micro-animations to collapsible toggle indicators
• Made all callout types collapsible
• Added page icons to Rustmark content

Fixed

• Collapsible elements now work in Firefox
• Nested callouts now render correctly

Changed

• Changed collapsible elements to use details blocks approach
• Various styling tweaks and improvements
• Improved features page documentation and examples
• Improved README documentation
• Improved CSS structure