Docs: Creating a contribution guide for Foundry

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I created a contribution guide for foundry that is meant to live both in the main directory and in the external documentation. Since this file lives outside of the docs folder but I still wanted it in the external documentation, the contributing_link.rst file is necessary. The content in the guide is based on the Atomworks contribution guide and some of the information provided in the Foundry README. Since this is mean to be rendered both on GitHub and Sphinx, I had to suppress Sphinx's xref_missing warnings because while these references are hidden in the Sphinx-built docs, they are visible in the GitHub rendered docs.

While creating this file I realized that a docs_requirements.txt file needed to be created so that contributors could easily make the necessary docs environment to run Sphinx.