docs: improve env files and doc

[themariofrancia]

fix(docs): resolve documentation inconsistencies across ATS guides

• Fix env var prefix: replace incorrect VITE_* with REACT_APP_* in ATS web docs
• Standardize .env file naming: rename .env.sample to .env.example across all packages
• Fix .env.local references: replace with .env (matching actual file names)
• Remove invented env vars (HEDERA_NETWORK, OPERATOR_ID, etc.) and point to .env.example
• Add testnet disclaimer note at the top of full-setup, quick-start, and deployment guides
• Add working directory context for deployment commands
• Remove duplicated "Step-by-Step Guide" section from deployment doc
• Move architecture content out of deployment guide (already in overview.md)
• Document NETWORK env var requirement and recommend deploy:hedera:testnet
• Document Hardhat Error HH8 (validates all private keys) with workaround
• Restructure .env.example: all network keys commented out by default to prevent HH8
• Remove unused deployed contract addresses from contracts .env.example
• Improve .env.example documentation across contracts, SDK, and web packages
• Clean up deployment guide: remove redundant sections, simplify expected output
• Remove duplicated env var section from sdk-integration.md
• Streamline full-setup guide: remove test/dev workflow sections, fix SDK step

[hedera-eng-infrastructure]

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