Make it clear which version the docs are for

[sravfeyn]

The current installation/setup docs are not clear on for which versions of the CouchDB the docs are about. For example from https://docs.couchdb.org/en/stable/setup/cluster.html

This section describes everything you need to know to prepare, install, and set up your first CouchDB 2.x cluster.

But further down in the docs, the docs mention notes about CouchDB 3.0. For example

CouchDB in clustered mode uses the port 5984, just as in a standalone configuration. Port 5986, previously used in CouchDB 2.x, has been removed in CouchDB 3.0.

As of CouchDB 3.0, steps 4 and 5 above are automatically performed for you when using the setup API endpoints described below.

As a first timer, it's hard to figure out which instructions are special to 2 or 3 and which ones are common.