Refine interface of Pipeline Syntax

[janfaracik]

Small one to make a few adjustments to the interface of Pipeline Syntax:

• Up link removed from sidebar
• 'Overview' now shows as a h1 - same as other pages
• Using jenkins-button class rather than YUI button

Before

After

Testing done

• Generating code works as before

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 demonstrates feature works or fixes the issue

[basil]

Causes jenkinsci/acceptance-test-harness#1619.