Jenkins Changelogs: flag changes which may require upgrade steps

[oleg-nenashev]

Right now Jenkins Weekly changelog does not explicitly link upgrade guidelines. For LTS we generate https://jenkins.io/doc/upgrade-guide/, but weekly users have to figure out upgrade steps on their own. I suggest to enhance logic so that we can improve the UX:

• https://github.com/jenkinsci/core-changelog-generator/ injects a flag based on the pull request labels: https://github.com/jenkinsci/jenkins/pulls?q=is%3Apr+label%3Aupgrade-guide-needed+is%3Aclosed
• changelog HAML puts a banner about upgrade steps if any changelog entry includes a flag
• LTS changelog points to upgrade guidelines in the banner
• Weekly changelog links upgrade guidelines in pull requests

Later we could generate upgrade guidelines in a a different way so that Weekly and LTS users could query upgrade guidelines between multiple releases. It is out of scope for this issue IMHO

[daniel-beck]

Would be nice to have a "GitHub diff" like feature that aggregates changes and related upgrade guides between any two versions, to answer the question "What do I need to know when updating from X to Y?"