Answer common Docker (+ docker-compose) questions

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

### What are the typical use cases?

* How can you run multiple Citus clusters at once?

Samay can share more about what the most common questions are. As long as they pertain to Citus and aren't merely basic Docker usage questions, we should include the answers in docs.

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

### Good locations for content in docs structure

Adding to https://docs.citusdata.com/en/v7.1/installation/single_machine_docker.html might not match the pages for other methods of installation, so we could make a new section? Maybe inside the Administration section.