Replace V3 docs with OpenAPI docs

[sethboyles]

We have an experimental OpenAPI spec/documenation here: https://cloudfoundry.github.io/cf-openapi/#description/more-resources

I'd be nice to replace our existing V3 documentation with these docs.

It'd be nice to somehow validate the accuracy of the OpenAPI docs, as well as merge in the docs to the core CCNG codebase so that new features do not need to be spread apart separate codebases.

There are also missing pieces of the current documentation that are not in the OpenAPI docs (like Valid Role Types)

Things to do before replacing the current docs

non-exhaustive list:

• Host openapidocs at something like openapi-docs.cloudfoundry.org
• Validate correctness of spec in some way.
• Add missing areas:
  • Valid Role Types
  • Concepts
  • Upgrade Guide
• Move OpenAPI docs to CCNG codebase

We might need to go section-by-section to compare what else is missing from the OpenAPI docs