Add Documentation for RecordData

[runspired]

We currently do not have documentation for this public feature.

[ChaiWithJai]

I think I can tackle this. We're looking to add information to this page?

[runspired]

We want to add an overview of the feature and how to use it here: https://api.emberjs.com/ember-data/release/modules/@ember-data%2Frecord-data

We would then add the RecordData interface (clearly marking it as v1, there is an unimplemented v2 that will arrive soon-ish) as well as document the DefaultRecordData.

[ChaiWithJai]

@runspired is my idea is to follow this API guide as a template.

And for my research, I'll read the Record Data RFC and grok the code.

[runspired]

I wrote docs for the store APIs associated with custom records and record-data implementations in #8114. RecordData itself still needs documented, I'm planning on adding documentation for it as I implement V2.

[runspired]

I added documentation when I implemented V2, improved documentation will be added when we ship the new Cache emberjs/rfcs#854