fix(core/message-bar): remove unwanted margin

[matthiashader]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (yarn build) was run locally and any changes were pushed
• Unit tests (yarn test) were run locally and passed
• Visual Regression Tests (yarn visual-regression) were run locally and passed
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Pull request type

Please check the type of change your PR introduces:

• Bug fix
• Feature
• Refactoring (no functional changes, no API changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

GitHub Issue Number: N/A

What is the new behavior?

Does this introduce a breaking change?

• Yes
• No

Testing

Other information

[nuke-ellington]

Since a list of message bars should have vertical margins of 0.5rem between the individual bars we should replace the
tags with actual margins in the examples.
This will better illustrate the usage to the users.
Also removing the built-in margins is kind of like a breaking change IMO?

[nuke-ellington]

In addition to the proposed changes all examples should have 0.5rem instead of 1.25rem as margin values.