Add a page explaining how to change the default "notification" URL interface

[lemeurherve]

Describe your use-case which is not covered by existing documentation.

There doesn't seem to be any page on jenkins.io explaining how to change the default "notification" URL interface to use the Classic UI or the Pipeline Graph View plugin for example instead of Blue Ocean.

Noticed via jenkinsci/pipeline-graph-view-plugin#245

Reference any relevant documentation, other materials or issues/pull requests that can be used for inspiration.

The only reference I found is https://docs.cloudbees.com/docs/cloudbees-ci-kb/latest/client-and-managed-masters/how-to-point-notification-links-to-classic-jenkins-path-rather-than-to-blueocean-endpoint

[KalleOlaviNiemitalo]

Currently, the system property or environment variable somewhat overrides per-user settings. See jenkins-infra/helpdesk#2833 (comment) and JENKINS-69006.

[lemeurherve]

I knew I forgot something, thanks for the reminder @KalleOlaviNiemitalo!

About this issue, getting a page explaining how users can set their prefered viewer would be nice already.

Regarding JENKINS-69006 I saw you've already found where the regression came, do you mind if I try to implement the fix you've proposed in https://issues.jenkins.io/browse/JENKINS-69006?focusedCommentId=428019, with you as co-author of course?

[KalleOlaviNiemitalo]

I actually started working on it last weekend, at https://github.com/KalleOlaviNiemitalo/display-url-api-plugin/compare/master...defer-config; but I referenced the wrong Jira issue and did not update tests yet.
If you can spend the time to fix the issue, that's great.

[dwnusbaum]

For what it's worth, I just fixed JENKINS-69006 in display-url-api 2.3.9, and I added a global configuration page to configure a default provider as well (JENKINS-43205) so that users don't need to set environment variables/system properties unless they really want to. See jenkinsci/display-url-api-plugin#202.

I also updated pipeline-graph-view to activate automatically by default if no preference is configured (matches the behavior of blueocean-display-url), see jenkinsci/pipeline-graph-view-plugin#29.