Add support for a markdown visualization type in dashboards

Much like a comment box on an issue, customers appreciate the ability to add commentary, links, and images to a space on their dashboards. That could be something like a link to runbooks, pointers to team members, slack channels, and a team logo, just to name a few. Having this content available with their service performance (o11y) and customer impact data (PA), means that if an issue arises, there's data available to help them take action.

A markdown widget could be added directly on a dashboard, no need to go into the viz designer, perhaps there are two buttons, one to add a new viz and another to add a markdown tile/widget?

Proposal

Iteration 1

• Add a new feature flag and a GlTabs with two tabs (Product analytics | Markdown) to the viz designer.

Iteration 2

• Explore existing markdown editors and see if we can reuse one for our needs.
• Add a markdown editor to the viz designer.
  • User enters markdown text and can save and preview the visualization.
  • On dashboards this renders as markdown HTML.
  • Consider making markdown a new "data source" that reads from the viz configuration.