Document Vectara JSON for API v2

[cjcenizal]

Idea

API v1 supported a Vectara JSON format: https://docs.vectara.com/docs/1.0/api-reference/indexing-apis/file-upload/format-for-upload. API v2 still supports a Vectara JSON format, but only in the Indexing API (#394). We need to add a similar page as the one I linked to, to educate folks about this format.

CC @nikron