Add real-time section to migration guide

[begriffs]

Communication goals (e.g. detailed howto vs orientation)

The migration section has content that applies to both MT and RT use cases, but the overall flow is geared towards MT. We should add a section in there specifically for RT.

Are there relevant blog posts or outside documentation about the concept/feature?

We used to have this page:

Real-Time Analytics Apps
========================================================================

In this model multiple worker nodes calculate aggregate data in parallel
for applications such as analytic dashboards. This scenario requires
greater interaction between Citus nodes than the multi-tenant case and
the transition from a standalone database varies more per application.

In general you can distribute the tables from an existing schema
by following the advice in :ref:`performance_tuning`. This will
provide a baseline from which you can measure and interactively
improve performance. For more migration guidance please `contact us
<https://www.citusdata.com/about/contact_us>`_.

The new page should be more informative.