docs: add upgrade and custom genesis instructions

[alxdca]

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Description

Add two how-to guides for Arbitrum Chain SDK v0.28.0:

• Custom genesis: Define account allocations, generate genesis files, configure the Rollup deployment, and initialize Nitro nodes consistently.
• Nitro contracts upgrades: Deploy, execute, and verify the supported 3.1.0 → 3.2.0 upgrade through the SDK CLI, including signing-key handling and multisig guidance.

Add sidebar entries and cross-links from the existing deployment, node configuration, SDK introduction, and upgrade documentation.

Document type

• Gentle introduction
• Quickstart
• How-to
• Concept
• FAQ
• Troubleshooting
• Reference
• Third-party content
• Codebase changes
• Not applicable

Checklist

• I have read the CONTRIBUTE.md guidelines
• My changes follow the style conventions outlined in CONTRIBUTE.md
• I have used sentence-case for titles and headers
• I have used descriptive link text (not "here" or "this")
• I have separated procedural from conceptual content where appropriate
• I have tested my changes locally with yarn start or yarn build
• My code follows the existing code style and conventions
• I have added/updated frontmatter for new documents
• I have checked for broken links
• I have verified that my changes don't break the build
• Third-party docs only: Do you agree to the third-party content policy outlined within CONTRIBUTE.md?
  • Yes
  • Not applicable

Additional Notes

The committed diff was inspected during PR preparation. Local build, link validation, and execution of the documented CLI workflows remain unverified.

Reviewers should confirm the SDK command examples and supported upgrade path, particularly the consistency requirements between custom genesis output, Rollup deployment parameters, and Nitro initialization.

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Project	Deployment	Actions	Updated
arbitrum-docs	Ready	Preview	Sep 11, 2026 1:37pm UTC