[SPARK-5310] [SQL] [DOC] Parquet section for the SQL programming guide

Also fixed a bunch of minor styling issues.

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Author: Cheng Lian <lian@databricks.com>

Closes #5001 from liancheng/parquet-doc and squashes the following commits:

89ad3db [Cheng Lian] Addresses @rxin's comments
7eb6955 [Cheng Lian] Docs for the new Parquet data source
415eefb [Cheng Lian] Some minor formatting improvements