Add product comparison to NebulaGraph documentation

[Subzeroo7]

Many friends in our community want to know the differences between NebulaGraph and other graph databases. I'll invite some NebulaGraph users to write comparisons of products like Neo4j, TuGraph, and HugeGraph. I hope this will help new users choose the graph database that's best for their business.

So add product comparison catalog to NebulaGraph documentation.

[CLAassistant]

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