Docs: Move "Implementing a plugin" to a better place.

[MichelDiz]

Documentation

Ref: https://docs.dgraph.io/query-language/#implementing-a-plugin

This session should be in another, more technical area. Instead, it is at the end of a session that is about the GraphQL+- language. As plugins are not part of this language. I see no reason for this session to be in query-language. And is more helpful if it is more visible.

Cheers.

[minhaj-shakeel]

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