doc: update node.js docs to mention layers

[astorm]

Updates the docs to point users at the Node Agents layer.

[astorm]

@bmorelli25 From a tech writing/elastic standards point of view, do you think we need to repeat the "how to add a layer in the UI" instructions a second time (as its written now), or can we assume the user will have picked this up from the initial section?

[ghost]

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• Start Time: 2022-03-09T17:04:03.357+0000

• Duration: 7 min 34 sec

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Test	Results
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Passed	120
Skipped	6
Total	126

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