Clean up and expand provided examples

[klauer]

The examples are old ones carried forward from iec2xml and don't really represent what blark can do nowadays.

I'd like to add:

• Better PLC code examples
• Examples on how to use blark in Python
  • Parsing plain source code
  • Parsing TwinCAT projects (with or without dependencies)
  • Using the summary tools
• One example on how to use the sphinx domain for documentation generation