How to use a custom pg data directory

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

### What are the typical use cases?

When installing on Ubuntu and other systems, it creates a cluster in a weird system directory.

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

Let people know they can use initdb wherever they like.

### 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?

* https://dba.stackexchange.com/questions/197809/changing-data-directory-for-postgres-on-centos
* Also Louise may have some extra details.

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