New API course article: Processes for external contributors

I added a new article to my API documentation course on processes for external contributors. One of the main advantages of a version-control-based system, especially using open-source technologies, is the promise of collaboration. Not just collaboration with your immediate team, but scaling beyond your team to also include other contributors within your organization and even contributors from the community. Many people embrace docs-as-code with the hope and expectation that many engineers will contribute to the docs. In this section, I cover processes to consider when external contributors (external to your team, not necessarily external to the company) write content.