Create clean-architecture.md

[Patrick Sweeting (psweeting1)]

What

Link to story

Patrick's findings from exploring clean architecture into tech docs

Checklist

Before you ask people to review this PR:

• Tests should be passing: ./gradlew test
• GitHub should not be reporting conflicts; you should have recently run git rebase main.
• Avoid mixing whitespace changes with code changes in the same commit. These make diffs harder to read and conflicts more likely.
• You should have looked at the diff against main and ensured that nothing unexpected is included in your changes.
• You should have checked that the commit messages say why the change was made.

[David Conneely (dconneely)]

YAML front-matter is missing source_url but looks good.

[David Conneely (dconneely)]

LGTM