[web] Added mermaid diagrams

The web site now supports mermaid.js diagrams. Added also a diagram on merge request label workflow.

To insert a diagram put it in a Markdown fenced code block with mermaid as the syntax, e.g.

```mermaid
graph TD;
  A-->B;
  A-->C;
  B-->D;
  C-->D;
```

Will render as:

graph TD;
  A-->B;
  A-->C;
  B-->D;
  C-->D;

Preview is here.

1. Feature description was added to the changelog