Applying the cookiecutter on an existing repo

Is Your Feature Request Related to a Problem? Please Describe

When @samcunliffe and I had to apply a cookiecutter to an existing repo, we ended up creating the template on a separate directory and copying files across. Is there a better way to do this? And should we document this usecase in this repo?

@matt-graham mentioned that in the ARC festival workshop they tried a different approach to keep the git history.

Describe the Solution You'd Like

Mention this case in the readme and add some steps with the best way to do this.

Describe Alternatives You've Considered

No response

Additional Context

No response

Review the README and the existing issue discussion, then compare the approach used in the ARC festival workshop for applying a cookiecutter while preserving Git history. Document the recommended workflow for applying the template to an existing repository, including steps for the reported use case; done when a newcomer can follow the README without copying files manually.