Add glossary terms/defs

At Saskia's suggestion I added some pretty nice glossary functionality, where defs are stored in a glossary page and appear on hover over the term - see https://docs.allenneuraldynamics.org/en/latest/policies_practices/publication_standards.html#data

Would be great to have a few of us make a more concerted effort to add more terms here. Just follow the existing pattern in glossary.md for the definitions, and then reference from other pages like
```{term}`intermediate result` ```

Once we have more entries we may also want to add a link to the top-level navigation