Write basic documentation for Kibana Development Platform

[joshdover]

Before 7.10 is released with first-class support for third-party KP plugins, we should have at least this docs on elastic.co/docs available to community developers:

• Basic "how to build a plugin" end-to-end example
  • Should outline how to use the provided tools (plugin generator, plugin helpers, plugin CLI) to create, package, and install a plugin
  • Should outline the basic structure of a plugin and how to interface with other parts of Kibana
  • Should include relevant links to other parts of the documentation or references
• remove any documentation related to the legacy system as that will not be supported in 7.10+

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[legrego]

Feel free to include me in this as well. This might be a good forcing function to make sure our feature controls docs are usable, for developers who wish to target the default distribution