feat(mermaid): add Mermaid.js diagrams plugin

[bartsmykla]

From: https://github.com/kumahq/kuma-website/pull/1547

The docs team recently added Mermaid.js to our own docs repo (see PR here) to make it easy to create diagrams. We also added some custom styles to match our branding colors. I thought it would make a nice addition to the Kuma repo, and I have a doc that I'm writing that would benefit from this diagram tool.

It can be used with the following syntax:

{% mermaid %}
{% endmermaid %}

With mermaid we can describe graphs and diagrams in a markdown file and have them render as what we want. So

{% mermaid %}
 gantt
    title Git Issues - days since last update
    dateFormat  X
    axisFormat %s

    section Issue19062
    71   : 0, 71
    section Issue19401
    36   : 0, 36
    section Issue193
    34   : 0, 34
    section Issue7441
    9    : 0, 9
    section Issue1300
    5    : 0, 5
{% endmermaid %}

would render as:

We've been using https://mermaid.live/edit to easily generate diagrams and see the rendered output before adding it to docs.

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