Add comprehensive multichain testing documentation

[Copilot]

This PR adds comprehensive documentation for Agoric's multichain testing framework, addressing the need for developers to understand how to test cross-chain applications.

What's Added

📖 New Documentation Page: /multichain-testing

• What is multichain testing - Comprehensive explanation of testing cross-chain applications
• Architecture overview - Details about the Starship-powered testing framework
• Prerequisites - System requirements, software dependencies, and knowledge requirements
• Resource requirements - Detailed table showing CPU/memory needs for different chains
• Setup instructions - Step-by-step guide with commands and verification steps
• Configuration options - Multiple config files for different testing scenarios
• Test suites - Organized test categories (main, Fast USDC, queries, staking)
• Example test scenarios - Real TypeScript code examples for common patterns
• Testing patterns - Asynchronous operations, error handling, state verification
• Best practices - Test organization, resource management, debugging, performance
• Troubleshooting - Common issues, debugging tools, and solutions
• Advanced topics - Custom configurations, relayer options, CI/CD integration

🔧 Navigation Updates

• Added "Multichain Testing" link in the Build section of the VitePress navigation
• Positioned after "End-to-End Testing" for logical flow

Key Features

The documentation covers:

• Multiple blockchain networks (Agoric, Osmosis, Cosmos Hub, Noble)
• IBC relayer testing (both Hermes and Go relayers)
• Real test examples with TypeScript code
• Resource planning with specific CPU/memory requirements
• Comprehensive troubleshooting guide
• Integration patterns for CI/CD pipelines

Visual Preview

The documentation provides developers with essential guidance for testing orchestration-based smart contracts that interact with multiple blockchain networks, making it easier to build reliable cross-chain applications on Agoric.

Fixes #1273.

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