[SPARK-2179] [SQL] Public API for DataTypes and Schema (Draft update for SQL programming guide)

This is the draft update for SQL programming guide. It adds doc for the data type and schema APIs. You can access it at http://yhuai.github.io/site/sql-programming-guide.html. 
It would be good to provide some motivation here.  Perhaps talk about programmatically creating a schema when it is not possible to statically define classes ahead of time.

Related: an example where the schema is determined statically might make more sense (i.e. read from the first row of the file?) but maybe that is too complicated...

Minor: Usually we just say "For example".