add a 'major features' episode

[tobyhodges]

Our immediate thought is that this episode would appear immediately after the 'Introduction to...' episode, and before we dig into the details of the episode structure. Candidates for inclusion in such a list of major features include:

• Instructor/Learner Views
• Accessible design/layout
• All-in-one, figures, and Instructor Notes pages
• Support for R Markdown and Markdown (with callout about quarto?)
• Integration with Glosario (when ready!)