Add API availability warning to the API documentation.

[someone13574]

Since there is a large number of people asking about a public API I feel as if a small warning at the top of the API docs to clarify that a public API isn't currently available is in order. It could additionally link to the FAQ question which goes into more detail.

It could read: "Open Assistant's API is for internal use only. However, the API code is available for hosting the model on your own hardware or with a cloud provider. See the FAQ for details."

[olliestanley]

Feel free to make a PR adding this