Update and review documentation about the setup of GeoNode vanilla with Docker for master

[giohappy]

Documentation here is outdated.

It must be updated as done for #238

Among other things it's referencing 3.2 and advising to use a docker-compose override instead of .env file

[jkariscodes]

Worked on the section referred by #241

[jkariscodes]

Updated as requested and review requested at #241