Adopt mdbook-alerts for Note comments

[rajsite]

In #149 Note: comments were made consistent as just comment blocks. In discussion https://github.com/bytecodealliance/component-docs/pull/149#issuecomment-2316155910 the idea was proposed to improve in the future.

Found the mdbook-alerts crate which adds support for the GitHub Flavored Markdown Alerts feature which could be used to address this.

[itowlson]

Those look really nice, and much more nuanced than the aside format. Great find!

[vados-cosmonic]

This is done! @itowlson / @kate-goldenring -- this should be closable :) we're using mdbook-alerts