PR workflow links to non-existent CONTRIBUTING.md

### Checklist

- [x] I added a descriptive title
- [x] I searched open reports and couldn't find a duplicate

### What happened?

The "create a PR" workflow has a link to CONTRIBUTING.md, but that URL doesn't exist.

```
...
Helpful links:
- Conda Org COC: https://github.com/conda/constructor/blob/main/CODE_OF_CONDUCT.md
- Contributing docs: https://github.com/conda/constructor/blob/main/CONTRIBUTING.md -->
```

### Additional Context

Has there ever been a contributing.md document?

Inspect the "create a PR" workflow referenced in the issue and locate its Helpful links section. Verify the CONTRIBUTING.md URL against the repository, then update the broken link as appropriate and confirm the workflow file still has valid YAML and a correct destination.

Written by the indexing model from the issue text.