Guide people with suggested pg config params

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

### What are the typical use cases?

When running Citus on prem people should adjust the default postgresql.conf parameters for better performance.

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

### 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