[SPARK-19684][DOCS] Remove developer info from docs.

This commit moves developer-specific information from the release-
specific documentation in this repo to the developer tools page on
the main Spark website. This commit relies on this PR on the
Spark website: https://github.com/apache/spark-website/pull/33.

srowen

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #17018 from kayousterhout/SPARK-19684.