Add general SQL workaround involving CTEs

[begriffs]

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

[onderkalaci]

Just FYI:

An example query/use-case we've been asked on github: https://github.com/citusdata/citus/issues/2321

Note that the original query that we need to rewrite is not a subquery, but a repartition outer join that Citus doesn't support.