Add "danger" and "note" CSS classes

[GuillaumeGomez]

After #106561, some people asked me if it would be possible to add "note" and "danger" equivalents. I think having multiple levels is a good idea so I added them. Here are screenshots of the results for each theme:

cc #79710

r? @notriddle

[rustbot]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @jsha

A change occurred in the Ayu theme.

cc @Cldfire

[jsha]

I think this is unnecessary complexity and maintenance burden. Also unnecessary complexity for the mental model of people reading the docs. What's the difference between a note and regular documentation? What's the difference between warning and danger?

[camelid]

An additional problem is that if we distinguish these levels based only on icon colors, it's not accessible to people who are color-blind or otherwise visually impaired.

[GuillaumeGomez]

Both your comments make perfect sense so I'll just close this. I'll provide my thoughts on how I came to do this:

I wanted to have a difference between "if you do that, things might not work" and "if you do that, it'll crash badly". At this point, I thought people might also want to highlight some information, hence the "note".

Thanks for the feedback!