Canonical source for shepherd writeup template is in data tracker repository

### Describe the issue

The shepherd writeup template has a note referring to https://github.com/ietf/chairs.ietf.org for the canonical source of the template.  The template is no longer located there.

### Code of Conduct

- [x] I agree to follow the [IETF's Code of Conduct](https://github.com/ietf-tools/.github/blob/main/CODE_OF_CONDUCT.md)