Docs: clarify how to always get the latest JMeter version

[ShirleyRex]

Description

This PR updates the documentation in xdocs/download_jmeter.xml to clarify how users can always find the latest JMeter release. A new section, “Getting the Latest Version”, has been added with references to:

• The official JMeter download page
• Maven Central
• GitHub releases

Motivation and Context

Users asked in issue #6499 about the best way to obtain the latest JMeter version immediately after release. This change makes the documentation clearer and provides direct links to trusted sources.

How Has This Been Tested?

• Verified that the updated XML is well-formed.
• Ensured the section renders correctly in the download_jmeter.xml structure.
• Checked that the links resolve to the latest available JMeter versions.

Screenshots (if appropriate):

Types of changes

• Bug fix - Documentation update (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.

Closes #6499