Documentation about merging arrays with YAML anchors

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When describing extends:, we say:

Arrays can be merged using YAML anchors.

… but we don’t really say how to do that in the section on YAML anchors, and it’s not entirely obvious from a quick web search.

It seems like this would be useful, especially since we now promote rules:, which behaves differently from only: and except: in that all the rules are in one array.