docs: fix README user documentation domain

[suyua9]

Summary

• update the README user documentation links from docs.teku.consensys.net to the currently reachable docs.teku.consensys.io domain
• keep the change scoped to the two README references to the user docs site

Testing

• verified both domains manually with curl; .io responds while .net currently fails
• not run otherwise (README link change only)

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Note

Low Risk
Low risk: documentation-only change updating README links, with no runtime or build impact.

Overview
Updates the README’s Teku user documentation links from docs.teku.consensys.net to the reachable docs.teku.consensys.io domain to prevent broken links for users.

^{Written by Cursor Bugbot for commit 016ace8. This will update automatically on new commits. Configure here.}

[github-actions]

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[suyua9]

I have read the CLA Document and I hereby sign the CLA

[suyua9]

recheck