Document how to make a new adapter via the template repo

Currently the OTIO documentation leads people to make new adapters in the contrib folder within the OTIO repo, but we have decided to move those adapters into separate repos. We should update the documentation to explain how to use the template repo: https://github.com/OpenTimelineIO/otio-plugin-template to make a new adapter.

Start by locating the current OTIO documentation that directs contributors to create adapters in the contrib folder. Read the otio-plugin-template repository and update the guidance to explain how to use it for a new adapter in a separate repository. Done means the documentation reflects the decided workflow and no longer directs users to create adapters in contrib.

Written by the indexing model from the issue text.