more examples and improvements to Note blocks

Issue in ./help/writing-essentials/using-markdown.md

Can you add "displayed" examples for Caution and Important note blocks? Also, I see that [!WARNING]" works too. Is that supported? I notice that they all seem to use the same blue "i" icon. And NOTE and TIP both have the same blue background. Can you use different icons and background colors for the different labels so they visually communicate more information?

Start with ./help/writing-essentials/using-markdown.md and review the existing Note block examples and label syntax. Determine whether [!WARNING] is supported and where the displayed icons and background colors are defined. Done means the requested Caution and Important examples are present, WARNING support is clarified, and the labels have distinct visual treatments.

Written by the indexing model from the issue text.