Fixes gh-1034 Package box is missing in the reference manual

doc/sphinx/book/box/index.rst

+.. _database-chapter:
+
 *******************************************************************************
                               Database
 *******************************************************************************

doc/sphinx/reference/box.rst

+-------------------------------------------------------------------------------
+                                Package `box`
+-------------------------------------------------------------------------------
+
+
+The box package has the database management component of Tarantool.
+All the functions for creating, maintaining, and monitoring Tarantool NoSQL
+databases are in the :ref:`Database chapter <database-chapter>`
+of the Tarantool User Guide.
+