docs(skills): migrate Arc documentation URLs to canonical docs.arc.io

[mehmetkr-31]

Summary

Updates all Arc documentation references across circlefin/skills and reference documentation from the legacy docs.arc.network domain (which returns 301 HTTP redirects) to the canonical docs.arc.io domain, and updates tutorial links to their active paths.

Changes

• Updated README.md link to https://docs.arc.io.
• Updated use-arc/SKILL.md LLM index to https://docs.arc.io/llms.txt.
• Updated swap-tokens/SKILL.md and swap-tokens/references/crosschain-token-movement.md App Kit & Swap Kit links to docs.arc.io.
• Updated bridge-stablecoin/SKILL.md App Kit & Bridge Kit links to docs.arc.io.
• Updated unify-balance/SKILL.md App Kit & Unified Balance Kit links to docs.arc.io.
• Updated use-gateway/SKILL.md and all its sub-references (deposit-evm.md, deposit-evm-browser-wallet.md, evm-to-evm.md, evm-to-solana.md, solana-to-evm.md) to https://docs.arc.io/arc/tutorials/access-usdc-crosschain.

Verification

• Verified all updated docs.arc.io URLs return HTTP 200 OK via curl.