Improve documentation for Wallet Connect

We have limited WalletConnect related documentation right now.
https://docs.metamask.io/guide/mobile-best-practices.html#the-provider-window-ethereum

Let's add steps on how dapps could primarily recommend MetaMask, and other integration information that could help dapp devs.

Start with the linked MetaMask mobile best-practices page, especially the WalletConnect provider-window section. Add clear steps for dapps to recommend MetaMask primarily and include useful WalletConnect integration information for dapp developers; the documentation should directly address those needs.

Written by the indexing model from the issue text.