Add documentation about running a packaged project

[noklam]

Description

• Add documentation about running a packaged project

Context

    We discussed this in **Technical Design** and talked about the following:

• The package_name argument provided in KedroSession.create() doesn't do anything, which is confusing to users. This argument should be removed, but since that is a breaking change it will happen in 0.19
• The workaround to run a run within a session is valid, but it's important to note that bootstrap_project is only meant for configuring a project that hasn't been packaged. When working with a packaged project configure_project should be used instead.
• The problem with the main() method is that it currently returns an exit code, so downstream processes can't do anything with it.

Originally posted by @MerelTheisenQB in #1423 (comment)