Add Base mainnet Merch Coin ERC20 example

[Bellafuria]

This PR adds a simple example of ERC20 Coin deployed on Base using thirdweb. Includes contract address and basic steps. Useful for Base Builders learning thirdweb Drops and Tokens. #BaseBuilders

Summary by CodeRabbit

• Documentation
  • Added an example guide for deploying an ERC‑20 coin on Base mainnet using the thirdweb Coin template; includes contract configuration (symbol, total supply) and step‑by‑step deployment via the thirdweb dashboard.

_{✏️ Tip: You can customize this high-level summary in your review settings.}