Add docs for INX plugins

[luca-moser]

Is your feature request related to a problem? Please describe.
With the upcoming stardust changes, some articles in the documentation may require updates and versioning. We need to be able to launch products with their respective docs to improve the overall developer experience.

Describe the solution you'd like
The following pages may require updating. Please feel free to add or remove any items you see fit.

Sections:

• Introduction:
  • What is INX?
    • Link to repository (protobuf definitions)
    • Link to example INX plugin
      Plugins:
• INX Chronicle
  • Ports and endpoints
  • Link to repository
  • Configuration
• INX MQTT
  • Ports and endpoints
  • Link to repository
  • Configuration
• INX Participation
  • Ports and endpoints
  • Link to repository
  • Configuration
• INX Spammer
  • Ports and endpoints
  • Link to repository
  • Configuration
• INX Proof-of-Inclusion
  • Ports and endpoints
  • Link to repository
  • Configuration
• INX Faucet
  • Ports and endpoints
  • Link to repository
  • Configuration
• INX Coordinator
  • Ports and endpoints
  • Link to repository
  • Configuration
• INX Indexer
  • Ports and endpoints
  • Link to repository
  • Configuration

A link to the parent issue can be found here: https://github.com/iotaledger/stardust-project/issues/45