Add docs for Microsoft.Extensions.VectorData.Abstractions package

[gewarren]

Describe the issue or suggestion

Needs API ref.

---

Associated WorkItem - 367794

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.