Adding docs for customHeader suppor for XHR call to Preid Server

[sanved77]

🏷 Type of documentation

• [ ] new bid adapter
• [ ] update bid adapter
• [x] new feature
• [x] text edit only (wording, typos)
• [ ] bugfix (code examples)
• [ ] new examples

📋 Checklist

• [x] Related pull requests in prebid.js or server are linked -> https://github.com/prebid/Prebid.js/pull/11780

🖼️ Screenshot

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