Add help remove last build

[AmoghParmar]

Fixes JENKINS-75463

This pull request adds missing online help documentation for the
“Remove last build” option of the LogRotator build discarder.

The help text explains the behavior, provides an example,
and references Pipeline usage.

Testing done

• Built Jenkins locally from source.
• Verified the new help icon appears for the “Remove last build” option
  in Manage Jenkins → Configure System → Build Discarder → Advanced.
• Clicked the help icon and confirmed the help text renders correctly.

Screenshots (UI changes only)

Before

• No online help was available for the “Remove last build” option.

After

• A help icon is displayed next to the “Remove last build” option,
  showing detailed documentation when clicked.

Proposed changelog entries

• Add online help for the “Remove last build” build discarder option.

Proposed changelog category

/label rfe,web-ui

Proposed upgrade guidelines

N/A

[StefanSpieker]

Thanks for the PR! I do not understand why you changed the dom.js. Can you elaborate on that?

[mawinter69]

Please make sure that when you work on a new issue to start from a clean state based on the jenkinsci master branch.

[mawinter69]

Please link to the github issue #16688 so that the issue is properly closed when merged. Jira is no longer used for core

[AmoghParmar]

Thanks for the PR! I do not understand why you changed the dom.js. Can you elaborate on that?

Thanks for the review.

The branch has been rebuilt from a clean jenkinsci/master state and now
contains only a single commit for the help text.
The accidental dom.js change is no longer part of the PR.

[AmoghParmar]

@mawinter69 Thanks for the guidance.

I have recreated the branch from a clean jenkinsci/master state and
updated the PR accordingly.

[StefanSpieker]

Thanks for the adjustments, but please fix the build, it is complaining about your file style:

13:11:34 [INFO] --- frontend:2.0.0:corepack (prettier) @ jenkins-parent ---
13:11:34 [INFO] Running 'corepack yarn exec prettier --check .' in /home/jenkins/agent/workspace/Core_jenkins_PR-26152
13:11:35 [INFO] Checking formatting...
13:11:46 [INFO] [warn] core/src/main/resources/hudson/tasks/LogRotator/help-removeLastBuild.html
13:11:58 [INFO] [warn] Code style issues found in the above file. Run Prettier with --write to fix.