Mention learner types theory

### How could the content be improved?

Issue suggestion based on trainee minute card feedback:
Add brief mention of (debunked) learner types theory (can reference the pre-reading) and state that thinking about different modes of learning still helps as part of UDL.

### Which part of the content does your suggestion apply to?

https://carpentries.github.io/instructor-training/09-eia.html#universal-design-in-learning-udl

Open 09-eia.html at the Universal Design in Learning section referenced in the issue and review the surrounding discussion and pre-reading. Add a brief mention that learner types theory is debunked while different modes of learning can still inform UDL, then confirm the section reflects the requested nuance.

Written by the indexing model from the issue text.