Historically, documentation was hosted using the GitHub wiki. The problem is that only Gobblin committers can modify the wiki; any external contributors who want to update or add documentation cannot do so. Editing the Gobblin Wiki is also not PR based, so any committer can make changes without going through a review process.
mkdocs.yml file. This file also specifies a master Table of Contents for the entire website.
ReadTheDocs is an open source, free tool that can build documentation in a GitHub repository and host it for public use. ReadTheDocs links to a specified GitHub project, and on every push to the repository the documentation is updated. ReadTheDocs essentially clones the repo and builds the documentation using either Sphinx or MkDocs (for Gobblin we only use MkDocs). It then hosts the documentation on internal servers so any end user can view the documentation. ReadTheDocs has other very nice features such as versioning of documentation, exporting documentation to PDFs, and search. ReadTheDocs is configured via its UI, the home page for Gobblin on ReadTheDocs is: https://readthedocs.org/projects/gobblin/. The full documentation for ReadTheDocs can be found here: http://docs.readthedocs.org/
For more information on how this architecture was decided and the different tradeoffs between other documentation services, check out the original PR: https://github.com/linkedin/gobblin/pull/788 and GitHub Issue: https://github.com/linkedin/gobblin/issues/724