Add additional types of admonitions

[atrnh]

Summary

We only have these pretty lame admonitions:

• Default
• Note
• Warning
• Hint

It'd be nice to have more. Like Sophia:

• Terms to know
• Big idea
• Did you know?
• Step-by-step

Details

We should also remove Note. Because it's so generic and useless. The final lineup:

• Terms to know
• Big idea
• Did you know?
• Step-by-step
• Warning
• Hint

[atrnh]

Some inspirational designs: