Add new info about job payload and dependencies

The documentation page was half completed, it missed the information how
to retrieve job payload and how to work with dependencies of jobs.
Fix inconsistent naming of namespaces and jobs

Every example uses the term "MyModule" and not "Example", so all module
namespaces are corrected to this one. Also, the job names have now more
descriptive naming used in other example pages as well.
Use consistent naming in service names of jobs

In other documentation pages, the send-email service is used and the
SendEmailJob, not the email-invoice job.
Rename page "internals" to "workers"

The page "internals" was supposed to describe the CLI interface and how
it tied to the controller to consume jobs. Essentially, this is the
workers part as explained by the introduction. Renaming to make this
more descriptive.
Write documentation page about workers

Add content how workers work, how the worker is started and how this
ties to an MVC worker controller.
Use the docs page and explain the events