Add User's Guide chapters on Data Definition and Data Manipulation.

Add User's Guide chapters on Data Definition and Data Manipulation.
Still needs to be filled with more information, but it gives us a
framework to have a User's Guide with complete coverage of the basic
SQL operations.  Move arrays into data type chapter, inheritance into
DDL chapter (for now).

Make <comment>s show up in the output while the version number ends in
"devel".

Allow cross-book references with entities &cite-user; etc.