Improve query performance tuning section

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

The section is fairly dense and contains historical cruft.

* "Table Distribution and Shards" is more like "Picking the best column for distribution"
* References older postgres versions, like the pg 9.0 performance book
* Add links to postgres docs for settings (e.g. shared_buffers)
* The postgresql tuning section contents should be separated better into subtopics
* EXPLAIN
* config params
* etc
* Add bulleted summaries to sections. The denser paragraphs will exist to support and explain the claims of the summaries.
* Add a diagram of push-pull execution to the CTE overhead section

### Good locations for content in docs structure

http://docs.citusdata.com/en/v7.5/performance/performance_tuning.html