Add real-time section to migration guide

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

The [migration](http://docs.citusdata.com/en/v8.0/develop/migration.html) 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:

```rst
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
`_.
```

The new page should be more informative.