Add Rediads user ID module documentation

[symplorpro]

🏷 Type of documentation

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

Description

This PR adds documentation for the new rediadsIdSystem Prebid User ID submodule.

Changes included in this PR:

• adds a new User ID submodule documentation page for Rediads
• documents installation and build commands for rediadsIdSystem
• documents publisher configuration for userSync.userIds
• documents expected bidRequest.userId.rediadsId and EID output
• updates the Rediads bidder documentation to indicate support for rediadsId

Files changed:

• dev-docs/modules/userid-submodules/rediads.md
• dev-docs/bidders/rediads.md

[netlify]

✅ Deploy Preview for prebid-docs-preview ready!

Name	Link
🔨 Latest commit	b921344
🔍 Latest deploy log	https://app.netlify.com/projects/prebid-docs-preview/deploys/69c3dbed6121cc0008d243a7
😎 Deploy Preview	https://deploy-preview-6503--prebid-docs-preview.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.