Clean up and consolidate alert shortcodes

[chalin]

We currently have two implementations, alert-md.md and alert.html (from Docsy). We should have only one:

• Eliminate alert-md.md - Alert cleanup #7090
• Use a copy of Docsy's new and improved alert.html (A better alert shortcode more markdown friendly google/docsy#941) - Alert cleanup #7090
• Cleanup alerts - Alert cleanup #7090:
  • Replace calls to alert-md.md by alert.html
  • Replace inlined alerts by calls to the alert.html shortcode, now that it can be invoked from includes, etc
  • Ensure that alerts are properly formatted when used in a Markdown list, per the new alert requirements
  • Drop color arguments for "info" and "primary", the shortcode will use the default "primary" - Alert cleanup #7090
• Drop our local copy of alert.html once Docsy is updated