Update URLs by replacing /contributing with /contributing/code (itera…

…tive#2657)

* Update URLs by replacing /contributing with /contributing/code

* Rename contributing/code to contributing/core

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,5 @@
 * [ ] Have you followed the guidelines in our
-      [Contributing document](https://dvc.org/doc/user-guide/contributing)?
+      [Contributing document](https://dvc.org/doc/user-guide/contributing/core)?
 
 * [ ] Does your PR affect documented changes or does it add new functionality
       that should be documented? If yes, have you created a PR for

CONTRIBUTING.md

@@ -1 +1 @@
-### See our contribution guide at [dvc.org](https://dvc.org/doc/user-guide/contributing#contributing).
+### See our contribution guide at [dvc.org](https://dvc.org/doc/user-guide/contributing/core).

README.rst

@@ -202,7 +202,7 @@ Related technologies
 
 Contributing
 ============
-Contributions are welcome! Please see our `Contributing Guide <https://dvc.org/doc/user-guide/contributing/>`_ for more
+Contributions are welcome! Please see our `Contributing Guide <https://dvc.org/doc/user-guide/contributing/core>`_ for more
 details.
 
 .. image:: https://sourcerer.io/fame/efiop/iterative/dvc/images/0