Move license to LICENSE.md

Description

This will allow Gitlab to automatically detect it and link to it from the front page of the project, which will make it more visible to users.

Also update the list of third party licenses (currently empty) to match and update the documentation to point to the new license files.

Checklist

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

This change does not affect any C/C++/Python/docs files, so no reformatting required.