Document stored packages overview

For now we have a few pages on the manual telling about PACKAGE related statements:

• CREATE PACKAGE
• DROP PACKAGE
• CREATE PACKAGE BODY
• DROP PACKAGE BODY

However, the manual misses an overview page similar to what we for stored procedures and functions.

The overview page should explain:

• What is a package. Why use packages.
• How packages are different from the SQL Standard modules.
• Package variables
• Public and private routines
• Executable initialization section
• Stored package privileges
• SHOW CREATE PACKAGE [BODY]
• SHOW PACKAGE [BODY] CODE
• SHOW PACKAGE [BODY] STATUS
• Name resolution order - resolving ambiguities:

  SELECT pkg.f1(); -- Schema `pkg`, schema function `f1`

  SELECT pkg.f1(); -- Current schema, package `pkg`, package function `f1`

• Stored package internals