Feature request: replace hard-coded default repository structure with cookiecutter template

We currently write a few files to new repositories such as 
```
".gitignore",
"README.md",
".cadet-rdm-cache.json",
".cadet-rdm-data.json",
"environment.yml",
"jupytext.yml",
"Dockerfile",
```

These could be placed in a cookiecutter template instead. This could allow users to replace the template with a custom template.