Include plugin class docstring in the online docs

[psss]

Let's allow providing an introduction text for each step plugin class so that general information can be included in the online documentation.

Plus, specifically for the provision step, let's make the hardware specification more discoverable. Also include a simple example at the very beginning of the hardware.

Pull Request Checklist

• [ ] implement the feature
• [ ] write the documentation
• [ ] extend the test coverage
• [ ] update the specification
• [ ] adjust plugin docstring
• [ ] modify the json schema
• [ ] mention the version
• [ ] include a release note