Resolves #10572

Pull Request Checklist

• Have you read How to write the perfect pull request?
• Have you read through the contributor guidelines?
• Have you referenced any issues you're fixing using commit message keywords?
• Have you added copyright headers to new files?
• Have you checked that both Scala and Java APIs are updated?
• Have you updated the documentation for both Scala and Java sections?
• Have you added tests for any changed functionality?

Helpful things

Fixes

Closes #10572

Purpose

Updates AssetsOverview.md with documentation on the behavior of the Last-Modified header. Includes an explanation of the issue, and a workaround.

Background Context

1. Built a Hello World application using Play Framework.
2. Added this line to build.sbt: ThisBuild / packageOptions += Package.FixedTimestamp(Package.keepTimestamps).
3. Built with sbt clean universal:packageZipTarball.
4. Deployed app and sent a GET request to {host}/assets/images/favicon.png and inspected the image headers. Verified the Last-Modified header is now correctly set.
5. Updated documentation.

References

• Last-Modified HTTP header of assets contained in jar is wrong when using sbt >= 1.4.0 #10572
• Reset timestamps to 1970-01-01 during packaging for repeatable builds sbt/sbt#5344
• packageTimestamp setting sbt/sbt#6237
• https://github.com/sbt/sbt/releases/tag/v1.5.0-M2