docs: add link to docs repo in PR template (#298)

Author: alexkrolick

.github/PULL_REQUEST_TEMPLATE.md

@@ -34,7 +34,7 @@ merge of your pull request!
 
 <!-- to check an item, place an "x" in the box like so: "- [x] Documentation" -->
 
-- [ ] Documentation
+- [ ] Documentation added to the [docs site](https://github.com/alexkrolick/testing-library-docs)
 - [ ] Tests
 - [ ] Typescript definitions updated
 - [ ] Ready to be merged