Sections about integrations: Go Client, Proxy endpoint

[hsanjuan]

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I think it's nowhere mentioned that we have a Go client for the API. Also, the proxy endpoint is only mentioned in the context of composite clusters but it's very useful without them.

We need either a full section about integrations, or (possibly better), several pages about it (one for the go-api client, one for proxy, one for using cluster as a go-library). With code examples.

[hsanjuan]

And finishing/doing rest API docs.