Content Guide for Docs

[alisondy]

Create a content guide for Flux docs

This will capture

• Where docs content resides
• What third party content is allowed on the docs, with examples
  • this will help clear up scope of use case documents

References https://kubernetes.io/docs/contribute/style/content-guide/

[dholbach]

Overlap with #131 ? Or can #131 be closed now that we distinguish between content and style guides?

[dholbach]

@alisondy What do you think? ^