[skip changelog] Fix missing links in documentation (#867)

Author: silvanocerza

Diff for: docs/CONTRIBUTING.md

@@ -350,3 +350,5 @@ If your PR doesn't need to be included in the changelog, please start the PR tit
 [prettier-website]: https://prettier.io/
 [prettier-vscode-extension]: https://github.com/prettier/prettier-vscode
 [npm-install-docs]: https://docs.npmjs.com/downloading-and-installing-node-js-and-npm
+[poetry-website]: https://python-poetry.org/
+[poetry-docs]: https://python-poetry.org/docs/