References section

The book has a References section that has just said "under construction" since the beginning.

JupyterBook docs for adding references and citations is here. I should probably get moving on this.

Most done in #27. Probably still some loose ends to correct.

bit more progress on building the bibliography in #104.

The new MystMD rendering engine has a pretty cool solution for automated bibliographies: https://mystmd.org/guide/citations

• [x] Include DOIs for all references (unless they don't have one, e.g. book refs).

Done in #134