Rewrite building & deployment guide with real-world lessons

[lamentierschweinchen]

Summary

• Rewrites references/building.md from a bare 73-line skeleton into a comprehensive 343-line deployment guide
• Based on actual deployment experience from the Bulletin Board and CLAO Ventures contracts

Key additions

• Rust version compatibility warning: Documents that Rust 1.83+ produces WASM incompatible with the Claws Network VM. Provides exact commands to install and use Rust 1.81.0.
• Pre-built WASM pattern: Teaches agents to check for deploy/ directories in GitHub repos when they cannot build locally.
• Mandatory flags: Every command now includes --proxy and --chain (the old guide omitted these, causing silent failures).
• Argument encoding reference: How to pass addresses, strings, numbers, BigUint, and hex bytes to constructor and endpoint calls.
• Gas limit table: Correct values for deploys (60M), simple calls (10M), and cross-contract calls (20M).
• Payable calls: How to send CLAW with a transaction using --value.
• Troubleshooting section: Covers invalid contract code, insufficient gas, oversized WASM, silent failures, and missing proxy/chain flags.
• Deployed contracts table: All 5 live contracts with addresses for reference.
• Project structure and release profile: Documents the wasm/Cargo.toml settings required for small binaries.

What was removed

• Reference to npx skills install multiversx/mx-ai-skills (not relevant to Claws Network deployment)
• Incorrect gas limits (old guide had 5M for calls, should be 10M minimum)
• Commands missing --proxy and --chain flags

Test plan

• Verify building.md renders correctly on GitHub
• Verify all clawpy commands include --proxy and --chain flags
• Verify Rust version guidance matches known working deployments

Generated with Claude Code

[BubuMVX]

This removes npx skills install multiversx/mx-ai-skills, is this an issue @michavie?
If you could evaluate this PR, it would be a great help.