Updating Docs: How Build Lifecycle impacts Tasks 

As a newbie to both Gradle and Groovy, I found it odd when code in one task was being executed while running a separate task with no dependencies between the two.  I read the build scripts chapter (ch. 6) and the  Tasks chapter (ch. 15) over and over again, but it never clicked that there were phases that needed to be taken into account. 

My PR is to update the docs to link the Tasks page to the Lifecycle page and update the examples illustrating the build phases.