Create OKX Kaia Explorer

[Iris3329]

Add OKX Kaia Explorer

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[scott-klaytn]

@Iris3329 Please change the file name from "OKX Kaia Explorer" to "okx-kaia-explorer.md" or something and add this file as a new entry (i.e. 'build/tools/block-explorers/okx-kaia-explorer') under 'build/tools/block-explorers/kaiascope' to the sidebars.js file.

[scott-klaytn]

Duplicated #151