[SQL][DOCS] Update sql documentation

Updated examples using the new api and added DataFrame concept

Author: Antonio Navarro Perez <[email protected]>

Closes apache#4560 from ajnavarro/ajnavarro-doc-sql-update and squashes the following commits:

82ebcf3 [Antonio Navarro Perez] Changed a missing JavaSQLContext to SQLContext.
8d5376a [Antonio Navarro Perez] fixed typo
8196b6b [Antonio Navarro Perez] [SQL][DOCS] Update sql documentation

Author: ajnavarro