[SPARK-19106][DOCS] Styling for the configuration docs is broken

## What changes were proposed in this pull request?

configuration.html section headings were not specified correctly in markdown and weren't rendering, being recognized correctly. Removed extra p tags and pulled level 4 titles up to level 3, since level 3 had been skipped. This improves the TOC.

## How was this patch tested?

Doc build, manual check.

Author: Sean Owen <sowen@cloudera.com>

Closes #16490 from srowen/SPARK-19106.