Update design of Design Library

[janfaracik]

Brings the Jenkins Design Library inline with the ongoing redesign of Jenkins, namely uses the core side panel heading class + the settings page container design.

Before

After

Testing done

• Pages look as expected

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed