Enable/fix markdown alert blocks + style rules.

[isoos]

• Supports GitHub Alerts with Markdown #7278
• styles rules and colors are copied from dartdoc verbatim
• pushed to staging for verification
• I think we should update the colors and styles, merging them with the already existing pub.dev styles. However I don't think it is worth to block the feature on it, it can be done after this is landed.
• I've commented out the icon block for now, adding a TODO to resolve it later, as pub.dev does not use Material Symbols Outlined fonts, as traditionally we've copied the required fonts into the static image directory. Let's discuss which direction we want to take.

[isoos]

/cc @parlough