Update Jenkins changelogs to always show a pull request link

[oleg-nenashev]

In the Jenkins pull requests we set a pretty high standard in terms of what goes to pull requests. We expect PR summaries to summarize changes, provide screenshots and upgrade guidelines if necessary. This information would be a good addition for advanced users, especially the upgrade guidelines part which is missing in LTS now... It could also encourage Jenkins users to participate in the discussions and, maybe, to submit their own pull requests.

Examples of pull requests where Jenkins users can benefit from PR links:

• Upgrade guide: [JENKINS 60920] header breadcrumbs CSS update jenkinsci/jenkins#4463
• UI details: Improve presentation of plugin categories jenkinsci/jenkins#4534

WDYT @MarkEWaite @daniel-beck @timja @res0nance?

[res0nance]

I think the idea is good although we might not want everything? Perhaps this idea could be trialed. For UI changes the screenshots offer a lot to the user to be able to see the new UI. But for Developer level changes it might not offer as much to an end-user

[daniel-beck]

Hiding the PR URL when there's a Jira issue was a deliberate choice for multiple reasons:

• Pull requests are an implementation detail. If we need them to explain a change, then we're doing the changelog wrong and should rather work some more on that, or the Jira issue (or require a Jira issue in the first place).
• I have absolutely no interest in having users (i.e. people with no intention to actually contribute, or to even spend a second to look at tags etc. to understand what is even the current status) comment on pull requests. See Winstone backport PR, auto refresh PR, or sidebar truncation PR. None of these comments were useful.

[oleg-nenashev]

FTR #2917 is likely a better and less controversial solution for upgrade guidelines