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.