Break up the spec into a set of documents

[pmackay]

Such that it can be displayed on a site like https://readthedocs.org/ or built into a doc set with a generated index.

This is a more significant change than that proposed by #464, but its a long spec and hard to digest currently.

[webron]

@fehguy - I've erased the link you created to the meta issue. This is not about splitting OpenAPI definition file into multiple ones, this is about splitting the specification document itself to sub-section documents.

[fehguy]

Sure thing--can you tie it to the appropriate meta issue?

[webron]

I'll create a meta documentation issue that isn't related to what the spec supports but rather how the spec is documented and the support documents we may want to provide.

[webron]

Parent issue: #589

[webron]

We've ended up deciding to not split the spec.