Extend user guide's domain model overview

[hankem]

This PR extends the user guide's domain model overview.

Old domain model overview


New domain model overview


To support links to javadoc.io, the PlantUML diagrams are rendered as SVGs, embedded as

<object type="image/svg+xml" data="domain-overview.svg" width="1989" height="982">
    <span class="alt">domain overview</span>
</object>

– which may not be supported by outdated browsers.