Document viem support for Hardhat Upgrades

[ericglau]

Documentation Pull Request

Summary

Updates the Hardhat Upgrades docs to cover using the plugin with viem, alongside the existing ethers usage.

• Using with Hardhat – install snippet now shows the plugin plus either the ethers or viem peer dependencies; adds "if you are using viem" sections for scripts and tests (import from @openzeppelin/hardhat-upgrades/viem, contracts by name, viem contract instances) and a short note on how it differs from ethers.
• Hardhat Upgrades API – reframes the intro for both ethers and viem, adds the viem setup/import, and notes viem's transaction-option differences.
• Migration guide – corrects the viem note and checklist to point at the viem API.

Emphasizes throughout that viem users must import from @openzeppelin/hardhat-upgrades/viem, since the examples otherwise use ethers.

Type of Change

• New documentation
• Documentation update/revision
• Fix typos or grammar
• Restructure/reorganize content
• Add examples or tutorials
• Update API documentation
• Other: ___________

Related Issues

OpenZeppelin/openzeppelin-upgrades#1279

Checklist

• Build succeeds locally with pnpm run build
• Lint is successful when running pnpm run check
• Docs follow OpenZeppelin Documentation Standards

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[ericglau]

Review criteria: (1) ethers content unchanged except for added viem mentions or wording generalized to cover both; (2) viem content mirrors ethers except for parameter-type differences, viem-specific options, or ethers-only options unsupported by viem; (3) content unrelated to ethers/viem unchanged; (4) every ethers-specific description or code block has an equivalent viem one with only the minimal required adaptations.

Review notes: For each function, compared the viem and ethers tabs against the actual plugin source (option types and signatures) plus the viem type-check test, confirming the option lists, parameter types (address vs. contract instance), return types, and address types all match the real API and stay consistent between the two tabs. Tabs are balanced, and the pages were previewed to confirm they render and the ethers/viem toggle flips everything together.