Improve query performance tuning section

[begriffs]

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

[mtuncer]

Explain output is diverged from current version. We should re-run and update here