Add CHANGELOG.md

Description

This collates all the change information from the Release Notes for all releases into a single document. Subsequent releases will have this information added in future.

We also update the Merge Request templates to tell contributors to update the Changelog.

Checklist

• Contribution meets RAL's licence terms
• [] make docs target runs successfully
• [] clang-format and clang-tidy run and changes included (C/C++ code)
• [] flake8 run and changes included (Python code)

There are no changes to any documentation files or any C/C++/Python files.