PR workflow links to non-existent CONTRIBUTING.md

[bkreider]

Checklist

• I added a descriptive title
• 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?