[New docs] Generate prop table JSON data

[tkajtoch]

Summary

We want to use react-docgen-typescript to generate JSON describing component props so that we can later use it for the new documentation website.

Acceptance criteria

• A new package @elastic/eui-docgen should be created and set up
• The package should use react-docgen-typescript and closely replicate the configuration we currently use for prop table generation to generate output JSON files