Document how to replicate reference tables to coordinator

Hi, we're preparing a POC to migrate an existing Postgres database to Citus for further scaling. We have a few tables we designated reference. They're also used together with many tables that will stay local to the coordinator. Some of the queries are very inefficient if they have to run in the distributed mode.

Therefore, I wondered why it wasn't possible to keep one reference table copy at the coordinator. I stumbled upon citusdata/citus#1615 and citusdata/citus#3155. Calling `master_add_node` indeed seems to solve our issues but I can't find any documentation about that function since v6. If I understand it correctly, it's an alias for `citus_add_node`, right?

It surprised me this behaviour of reference tables wasn't the default. Are there any drawbacks to calling `citus_add_node` with the coordinator address and `groupId => 0` apart from the reference tables storage requirements? It would be great if this use case was documented.

Start by reviewing the current documentation for master_add_node and citus_add_node, then read the referenced issues citusdata/citus#1615 and citusdata/citus#3155. Document how to replicate reference tables to the coordinator, including whether citus_add_node with groupId => 0 is the supported equivalent and what drawbacks apply.

Written by the indexing model from the issue text.