Add dedicated function to generate doc archives from external module

With this utility function one can generate doc (.qch) archive from
given module. Reads the following arguments from environment:
- qt5 archive uri
- pre-built icu archive for given qt5 package
- module src package for which to generate the docs

Change-Id: I875864bd1579d26344987ac3cd057819852d5eb5
Reviewed-by: Antti Kokko <antti.kokko@digia.com>