New “backups and replication” section

## Why are we implementing it? (sales eng)

### What are the typical use cases?

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

* Talk about the two modes of replication: citus / streaming
* Pros/cons
* Discourage citus replication for MT
* For RT workloads which use batch loading it’s OK
* We’ll need to iterate during the PR about specific scenarios in which it might be OK
* High throughput ingestion can have problems in citus replication
* See SE doc for details
* Remove from citus replication from prominent places in the docs such as Creating and Modifying Distributed Tables (DDL) where we introduce the replication factor. Those sections oversell that form of replication which is not appropriate for every scenario.

See also https://github.com/citusdata/citus_docs/issues/502

### Good locations for content in docs structure

## How does this work? (devs)

### Example sql

### Corner cases, gotchas

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

### Link to relevant commits and regression tests if applicable