Document V1 API

[Sytten]

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Description

Currently the V1 API is considered an internal implementation detail, but we want to open it up so people can build on it.
We should make it clear that we won't do breaking changes to it, but we don't promise how long it will be used either.

Steps

• document the JSON format
• document the compression (gzip)
• document the auth
• document the current limitations (reporting service is pretty dumb)
• document the DDSketch
• document the operation signature creation