update decorators docs

[JLHwung]

Resolves https://github.com/babel/website/issues/2768

In this PR we

• updated the examples of decorators to the "2023-01" version
• marked the version option as required
• removed the CLI usage because this plugin requires at least one plugin option, which can not be passed from command line
• linked the spec repo for a brief summary of different decorator versions
• recommend to use preset-env when using with class properties / private methods transform, c.f. https://github.com/babel/babel/discussions/15582

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