Improve Upgrading JIRA Safely page - phase 2

Further improve Upgrading JIRA Guide from marketing/usability viewpoint.

Feedback points from chat with Ben:

• Paragraph notes (blue sections) are very wordy in Overview section, especially in lower resolution monitors. Consider streamlining and breaking up text, much like white-space text.
• Consider adding callouts throughout document, e.g. need help, contact our support team here.
• Consider adding more diagrams.