Consider adding router executor back to Citus Query Processing section

In [Citus 5.0](http://docs.citusdata.com/en/v5.0/performance/query_processing.html), we used to have a section on Router Executor in our Citus Query Processing section. Currently, [the page](http://docs.citusdata.com/en/v5.2/performance/query_processing.html) doesn't have that section since we removed it with 5.1. However, since the multi-tenant use case uses it heavily, we should probably re-add the section.