## What changes were proposed in this pull request?

See JIRA for the motivation. The changes are almost entirely movement of text and edits to sections. Minor changes to text include:

- Copying in / merging text from the "Useful Developer Tools" wiki, in areas of
  - Docker
  - R
  - Running one test
- standardizing on ./build/mvn not mvn, and likewise for ./build/sbt
- correcting some typos
- standardizing code block formatting

No text has been removed from this doc; text has been imported from the https://cwiki.apache.org/confluence/display/SPARK/Useful+Developer+Tools wiki

## How was this patch tested?

Jekyll doc build and inspection of resulting HTML in browser.

Author: Sean Owen <sowen@cloudera.com>

Closes #13124 from srowen/SPARK-15333.