Update "Tip" icon in the Notice block

[ndiego]

The "Tip" icon in the Notice block is a question mark ?.

This does not exactly fit with a "Tip" and would be better suited for a question. I am thinking we update this to a lightbulb icon. This would establish consistency with the GitHub markup options.

Note

Highlights information that users should take into account, even when skimming.

Tip

Optional information to help a user be more successful.

Warning

Critical content demanding immediate user attention due to potential risks.

Caution

Negative potential consequences of an action.

@WordPress/meta-design thoughts? Perhaps we use the tip icon in the icon package.

[jasmussen]

I'd be here for the lightbulb!