docs: Refactor examples

[Sherlock113]

What does this PR address?

@parano @ssheng As discussed before, this pr does the following:

1. Maintain the full example list on the index page: https://docs.bentoml.com/en/latest/use-cases/index.html. It should be the single source of truth for users to find examples.
2. Put the index page link in the readme in the examples folder to redirect readers.

What's next:

1. Migrate the remaining examples to separate repos: MLFlow, Gradio, SKlearn, XGBoost. @larme
2. The rest will be deleted. @Sherlock113

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