Update quickstart to include Responder for RaftTypeConfig

[danthegoodman1]

What kind of doc do you ask for?

• [ ] Code comments.
• [ ] Architecture design.
• [ ] Algorithm explanation.
• [x] Guide.
• [x] Examples.

Describe the information you want Missing the Responder type for the example in https://docs.rs/openraft/latest/openraft/docs/getting_started/index.html (RaftTypeConfig).

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[drmingdrmer]

The Responder type is optional and can be left absent. These has been added to getting-started.md in:

• 1181