Explain middleware

[sdirix]

• Add a "middleware" dcoumentation page under "docs"
• Conceptional explanation:
  • Modify actions within JSON Forms, for example for "derived data manipulation"
  • Warning: Be careful as this is powerful however invalid changes will break JSON Forms
  • Warning: This is very deep in the internals, so there might be changes in future JSON Forms versions
• Explain the concept of the middleware
• Showcase the most important actions to customize (init, updateCore, update)
• Showcase two (functional) example middlewares ("controlled mode" and "derived data manipulation")