docs: add Building Onchain Games on Ink tutorial (#613)

[schoolkamsergj]

Summary

Adds a new tutorial, Building Onchain Games on Ink (Phaser 3 + wagmi), under build/tutorials. It walks developers through connecting a Phaser 3 game to a smart contract deployed on Ink, using wagmi + viem for wallet connection and onchain score submission.

Closes #613

What's included

• New page: src/pages/build/tutorials/building-onchain-games.mdx
• Sidebar entry added in src/pages/build/tutorials/_meta.json
• Covers: project scaffold, a ScoreBoard Solidity contract + Foundry deploy to Ink Sepolia, wagmi config for ink / inkSepolia, a Phaser ↔ @wagmi/core bridge, and transaction-UX tips for games.

Notes

• Uses existing Ink network details (mainnet 57073, Ink Sepolia 763373) and links to the existing Foundry, faucets, and SuperchainERC20 pages.

[schoolkamsergj]

Hi — this PR is currently blocked on "2 workflows awaiting approval", so none of the required checks (build, format, js-lint, md-lint) have been able to run yet.

Could a maintainer approve the workflow run so CI can report status? Happy to fix anything the linters flag.

For context: this closes #613 (a docs gap I opened in May), adds one new tutorial page under build/tutorials plus a _meta.json sidebar entry. Commit is signed/verified. Thanks!