Add `mermaid` generation of diagrams for markdown

[sentenz]

mermaid generation of diagrams like flowcharts or sequence diagrams from text in a similar manner as markdown.

Command line tool for the Mermaid library:

npm install -g @mermaid-js/mermaid-cli@9.1.7

Adds Mermaid diagram and flowchart support to VS Code's builtin markdown preview:

• Markdown Preview Mermaid Support