Use better default callout icons

[AThePeanut4]

In my opinion, the current default callout icons look a bit out of place when compared to e.g. the default heading icons. The note icon looks a bit too big, and the gap between the icons and text is also bigger (a lot bigger for the Tip callout).

This changes all the callout icons to use the nf-md-*_outline icons (https://www.nerdfonts.com/cheat-sheet), which are already being used for the Important and Warning callouts. Because they are smaller icons, the gap between the icon and the text can be reduced to generally be the same as for the headings.

An alternative is to use the same icons GitHub uses - the nf-oct-* icons. These are larger icons, so the gap between the icon and text can't be made smaller without losing the gap entirely for all but the Tip callout. The very large gap for the Tip callout is the main reason I prefer the nf-md-*_outline icons instead.

This is all obviously personal preference, so I completely understand if you don't agree - feel free to close this if you'd like.

Before

	Text	Icon class
Note	 Note	nf-oct-info
Tip	 Tip	nf-oct-light_bulb
Important	󰅾 Important	nf-md-comment_alert_outline
Warning	 Warning	nf-cod-warning
Caution	󰳦 Caution	nf-md-alert_octagon_outline

After

	Text	Icon class
Note	󰋽 Note	nf-md-information_outline
Tip	󰌶 Tip	nf-md-lightbulb_outline
Important	󰅾 Important	nf-md-comment_alert_outline
Warning	󰀪 Warning	nf-md-alert_outline
Caution	󰳦 Caution	nf-md-alert_octagon_outline

Alternative

	Text	Icon class
Note	 Note	nf-oct-info
Tip	 Tip	nf-oct-light_bulb
Important	 Important	nf-oct-report
Warning	 Warning	nf-oct-alert
Caution	 Caution	nf-oct-stop

[MeanderingProgrammer]

I appreciate the input!

I was pretty loose with the icon selection and hadn't noticed the inconsistency between nf-oct-*, nf-md-*, and nf-cod-*. Standardizing everything in terms of nf-md-* makes a lot of sense to me.

Font rendering also plays a large role here, so different users will end up with different experiences. For instance mine look like:

Before	After	After + Extra Space

The caution icon renders relatively smaller with your font setup compared to my own, and I'm sure others will have different minor variations.

I think of these 3 I prefer the option you selected as well so will merge this in and update the documentation.

Thanks for the PR!