Doc: Add use case, RobustMQ

[socutes]

RobustMQ also uses OpenRaft to build clusters, which we hope will add to this use case.Thank you. please evaluate whether it is appropriate.

Thank you very much.

Checklist

• Updated guide with pertinent info (may not always apply).
• Squash down commits to one or two logical commits which clearly describe the work you've done.
• Unittest is a friend:)

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This change is 

[drmingdrmer]

Thank you! This is another excellent use case. Appreciate it.