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

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.

Doc build, manual check.

Author: Sean Owen <sowen@cloudera.com>

Closes #16490 from srowen/SPARK-19106.

(cherry picked from commit 54138f6e)
Signed-off-by: Sean Owen <sowen@cloudera.com>