Replace COI.md link w/ link to COI on RTD
Replace COI.md link w/ link to COI on RTD

- The read-the-docs link to COI.md on github was broken.
- The `.md` file extension was being stripped leading to a 404 error.
- Replace the link to GH markdown with the COI as documented on the joss read-the-docs site
openjournals/joss,openjournals/joss,openjournals/joss