Improvements to the Step by step tutorial

[tamararivera]

I'm reviewing the Getting started section as part of my onboarding. This PR is for changes to the Introduction to ServiceBus tutorial.

Observations:

• What is the name of the tutorial? NServiceBus Step-by-step or Introduction to NServiceBus
• The parts were called Lessons, I decided to follow the other tutorials and renamed them Parts
• There is a partial for the introduction text that is repeated on the first part, I'm tempted to remove it
• This line on part 2 confused me: 'Since the handlers in the tutorials are very simple and mostly just log information, they don't need to have the async keyword in the method definition. However, it's possible to add it and modify the handler to not return a Task'
• Part 2 takes more time to read than 15-20 minutes
• On Part 3, we could remove the note about Async Main, as it from a blogpost from 2017
• What date should we put on the review date?