docs: add evm contract verification guide

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Description

Adds /docs/evm.md, syncing the referenced OpenAPI-related documentation content for EVM contract verification on Subscan (Remix metadata JSON workflow).

Original data:

• Reference commit: 3402fc769368e6855b81a3fb14c1969eee27cf01
• Added file: docs/evm.md

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Para ID Owner & Signature Account:
N/A (documentation-only change)

Data that needs to be signed:
docs/evm.md

Signature Hash: N/A (documentation-only change)