PG 11 partitioning support

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

### What are the typical use cases?

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

What features do we also support in citus? One in particular Samay mentioned is having unique indices in a parent table for use with upserts.

### 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://h50146.www5.hpe.com/products/software/oe/linux/mainstream/support/lcc/pdf/PostgreSQL_11_New_Features_beta1_en_20180525-1.pdf

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