Add general SQL workaround involving CTEs

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

### What are the typical use cases?

Complex subqueries that Citus refuses to run.

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

Explain how rewriting certain subqueries as CTEs is a workaround for complex subqueries.

TODO: Sai will provide examples of queries that don't work, and their rewritten forms that do.

### Good locations for content in docs structure

http://docs.citusdata.com/en/v7.5/develop/reference_workarounds.html

## 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