assistant: Refine settings view's instruction visuals (#25812)

I've been bothered by using simple hyphens for bullet lists here for a
while; it kinda looked cheap and not well-formatted. So, in this PR, I'm
adding a new, custom UI component in the `language_models` crate, called
`InstructionListItem`, based off the `ListItem` that's somewhat
mimic'ing what a `<li>` would be on the web.

It does have a "rigid" structure as in it's always a label followed by a
button (which is optional), but that seems okay given it has been the
overall shape of the copy we've been using here. Also, never really
loved that we were pasting URLs directly, that kinda felt cheap, too. I
could see an argument where it's just clearer, but it looks too
cluttered, as URLs aren't super pretty, necessarily.

| Before | After |
|--------|--------|
| <img
src="https://github.com/user-attachments/assets/ffd1ac27-b1f4-450d-abf5-079285fc9877"
width="700px" /> | <img
src="https://github.com/user-attachments/assets/28fb9d0d-205d-45d8-9e43-1aaa947adc96"
width="700px" /> |

Release Notes:

- N/A