Add Arc mainnet API and launchpad documentation

[buddies2705]

Summary

• add Arc mainnet guides for trades, launchpads, transfers, events, calls, transactions, and balances
• add checked launchpad contracts and saved Bitquery IDE examples
• add Arc mainnet to the sidebar, endpoint list, chain directories, homepage, and search
• point every Arc testnet page to its matching mainnet guide
• update GraphQL Trading coverage from nine chains to ten after checking Arc live
• tune titles and descriptions using Ahrefs keyword data

Checks

• 64/64 GraphQL examples checked against production
• Arc returned data in Trading.Trades, Trading.Tokens, and Trading.Pairs
• yarn build
• yarn check-links
• 64 saved-query records and IDE links audited
• Ahrefs keyword and on-page SEO review
• rendered testnet-to-mainnet links checked
• repo-wide Arc discovery audit