Enable mermaid

[lionello]

Per https://docusaurus.io/docs/next/markdown-features/diagrams

https://docs.defang.io/docs/intro/how-it-works has a mermaid diagram that's not shown.

[commit111]

currently, the image is fixed but the code for it has some slight errors. the docusaurus.config.js file expects two more arguments for markdown (which is where mermaid is enabled): format and mdx1Compat