Add description for plugin manager

[zbynek]

Description

See JENKINS-68300. This adds missing description for Jira plugin and makes sure description is exported for JDL and core-js plugins.

To verify you can compile all plugins, index.jelly should appear in the root of the .jar file.

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description
• [n/a] Appropriate unit or acceptance tests or explanation to why this change has no tests
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verified the change matches the issue description
• Reviewed the code
• Verified that the appropriate tests have been written or valid explanation given