Add v15.3.0 release notes

[JoeJimFlood]

Proposed changes

This pull requests adds the release notes for v15.3.0 to the documentation.

Impact

The v15.3.0 release notes will be available when the documentation is viewed.

Types of changes

What types of changes does your code introduce to ABM?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

How has this been tested?

Please describe the tests that you ran to verify your changes.

• Viewed in preview mode in GitHub editor

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings