Add Datamodel spec 2.0.0

[sajith]

To convert the original spec to reStructuredText, I downloaded a .docx of the original and used pandoc:

pandoc --extract-media . spec.docx -o spec.md

And then made some small tweaks by hand.

[YufengXin]

Does the build error msg mean anything?

[sajith]

Does the build error msg mean anything?

Yes, it sort of does. We're running sphinx-build -W, asking Sphinx to treat warnings as errors. I did not remove a duplicate label from the new spec (the other label is in the previous version of the spec), and sphinx-build is complaining about that.