Add Chain APIs tab and update navigation

[0xmts]

Summary

• rename the Node tab to "Node RPC" and add a new "Chain APIs" tab in the header
• move the chain-specific sidebar sections into the Chain APIs tab while reusing the Ethereum section in both tabs via a shared anchor
• update the chain APIs overview links to reference the new Chain APIs routes

Testing

• not run (not requested)

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https://chatgpt.com/codex/tasks/task_b_68f1328b4b308329bf6f183849dabdda

[github-actions]

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