fix!: Update alert appearance, make ‘warning’ the default and icon non-optional

[VincentSmedinga]

• Updates to latest design: background now white, corrected text levels
• Adds a border token; not sure if the current set is exactly correct
• Uses Heading component to display title
• Adds ‘warning’ severity and makes it default
• Removes icon prop – distinguishing colour alone is not enough
• Hmm, this could / should have been 3 separate PRs
• Extends and organizes documentation, although not complete yet