Update, rework, and extend docs

[timebertt]

This is the last PR in the 5-part series for implementing the new external sharding approach (ref #31).

This PR tackles:

• immense rework and extension of docs
  • there is now a lot of documentation about getting started with the project, implementing sharding in your own controller, understanding the design, etc.
  • update the architecture diagram for the external sharding implementation
• cmd/shard is now the reference implementation for a shard
  • its components have been made reusable in pkg/shard to give controllers based on controller-runtime a head-start for implementing sharding

With this PR, the project is ready to be announced publicly. 🎉
The docs should now explain the project's purpose much better and how folks can start using sharding in their own controllers.