Add integration page (WIP)

[janfaracik]

Draft PR to add guidance on adding items to the Jenkins header, also serves as a new home for guidance on other integrations in to Jenkins.

jenkinsci/jenkins#10729

Testing done

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