Add Boldwin Rapid Bidder Adapter documentation

[BoldwinDev]

🏷 Type of documentation

• [x] new bid adapter

PR for the adapter: https://github.com/prebid/prebid-server/pull/4478

📋 Checklist

• [ ] Related pull requests in prebid.js or server are linked. https://github.com/prebid/prebid.github.io/pull/6202
• [x] For new adapters check submitting your adapter docs

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[muuki88]

Can you link the accompaning PR for those docs?

[BoldwinDev]

Redundant info is removed. The link to the PR is added