Change log entry for the new change notifications feature #18532

Description:

Adds a change log entry briefly describing the new "What's new?" / change notifications feature added by #16805 and a link to the developer guide.

New Change Notifications

• Plugins can now provide a list of changes which will be displayed as part of the "What's New?" menu notification. Learn more about how this works in the developer guide.

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed

This issue is in "needs review" but there has been no activity for 7 days. ping @matomo-org/core-reviewers