Support Mermaid.js in our docs & blog markdown

[huan]

GitHub has been supporting Mermaid.js officially in the markdown files: Include diagrams in your Markdown files with Mermaid

It would be great to add support to our website so that Mermaid.js can be used in the markdown files out-of-the-box.

Related links:

• Embed Mermaid in Jekyll without plugin
• https://github.com/facebook/docusaurus/issues/1258

thanks to @willin for the links!