[docs] Custom Plugin #1686

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I am trying to implement a dock panel for adding new nodes to the graph by d&d e.g. and I found there are APIs for adding plugin to the graph here: https://g6.antv.vision/en/docs/api/Graph#addpluginplugin

While I cant find any docs on how to implement your own plugin, is there any related reference in the docs other than the existing code base? Thanks!