Make time zone list expandable in help

[AmoghParmar]

Fixes #20780

This pull request improves the Build periodically help documentation by replacing the extremely long in-page list of Java time zone IDs with a link to the official time zone reference.

Previously, the help popup displayed hundreds of time zone entries, making the help panel unnecessarily long and difficult to read. Most users do not need to browse the full list directly inside Jenkins. The updated help text now links to the official TZ database reference instead, keeping the help concise and more user-friendly.

Testing done

This change affects only help documentation UI and does not modify any runtime logic.

Manual testing steps:

• Built Jenkins locally from source
• Started Jenkins using the locally built WAR
• Opened a Freestyle job → Configure
• Enabled Build periodically
• Clicked the help icon next to the Schedule field

Verified:

• The large embedded list of time zone IDs is no longer displayed
• A documentation link to the official time zone list is shown instead
• All other help content renders correctly
• No layout or formatting issues in the help popup

Since this change only updates static help content, no automated tests were added.

Screenshots (UI changes only)

Before

The help popup displayed a very long list of Java time zone IDs directly in the page.

After

The help popup now shows a short explanation and a link to the official time zone documentation instead of embedding the full list.

Proposed changelog entries

• Make time zone list expandable in help.

Proposed changelog category

/label rfe,web-ui

Proposed upgrade guidelines

N/A

Submitter checklist

• The issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate and in the imperative mood.
• There is automated testing or an explanation as to why this change has no tests.
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
• New deprecations are annotated with @Deprecated(...), if applicable.
• UI changes do not introduce regressions with Content Security Policy.
• For dependency updates, there are links to external changelogs.
• For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@jenkinsci/core-pr-reviewers

[janfaracik]

Could you post a Before picture? Thanks.

[AmoghParmar]

Could you post a Before picture? Thanks.

Thanks @janfaracik for the suggestion!
I’ve now added a Before screenshot to the PR description so the difference between the old long time zone list and the updated help with the documentation link is clearly visible.

Please let me know if any other changes are needed.

[daniel-beck]

This makes this help actively unhelpful. Previously you knew exactly what was supported, now it's trial and error.

If the list is too long, wrap it in a <details> or something, so it's only shown after another click.

[daniel-beck]

Fixes #60635

We migrated from Jira to GH issues. This issue used to be JENKINS-60635 and is now #20780. Please use the correct ID.

[MarkEWaite]

Tested interactively and the change looks good to me. I like the strong formatting on the clickable link to expand the list so that readers are more likely to see the list.

[MarkEWaite]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback.

/label ready-for-merge