Glossary: update and use more effectively

During work on the course over the past year or so, we did not keep the glossary up to date.

- [ ] e.g. “array broadcasting” or “vectorisation” are missing
- [ ] full read-through of course, to catch other missing terms
- [ ] What’s the best way to highlight glossary terms throughout the course material?

Actually, the third point is probably the one we should tackle first—if the glossary isn’t clearly highlighted throughout the main text, most learners will never know about it and contributors/maintainers will regularly forget to update it.