Post Workbench transition improvements

- [x] Move over to using Mermaid within the lesson once it is enabled in the workbench (link to
current MR for that)
- [x] Add alt-text to all images and consider captioning some of them. The caption is now in the
place where the alt-text used to be, and the alt-text is in an extra attribute after the
definition of the image (e.g. [caption text](./path/to/image){alt='Alt text'})
- [x] Consider creating learner profiles
- [ ] The "More" drop down to get to learner extras moves into the hamburger side menu at the
bottom when the screen width is reduced beyond a certain limit. Doug and I felt this wasn't great
UX and could possibly be improved.