Add MetaMask Embedded Wallets onboarding section (#662)

Added section on MetaMask Embedded Wallets for onboarding.

Author: yashovardhan

docs/base-chain/tools/onboarding.mdx

@@ -20,6 +20,12 @@ description: Documentation for various ways to onboard users in your apps on Bas
 
 Use [Auth Guide](https://www.openfort.xyz/docs/guides/javascript/auth) to allow several onboarding methods into your game regardless of the platform. 
 
+## MetaMask Embedded Wallets
+
+[MetaMask Embedded Wallets](https://metamask.io/developer/embedded-wallets) (formerly Web3Auth), brings instant, secure, & seamless OAuth-based and Web3 Native onboarding to Base. Users can log in with Google or Apple and get a non-custodial wallet instantly connected to the network — with no setup or seed phrase required. Setup is entirely handled in the MetaMask (Web3Auth) Dashboard — developers just enable Base, and it works out of the box.
+
+Learn more at [docs.metamask.io/embedded-wallets](https://docs.metamask.io/embedded-wallets/).
+
 ## Privy
 
 [Privy](https://www.privy.io/) is a library designed for progressive user onboarding and authentication. It enables users to connect to your app using traditional methods such as email addresses, phone numbers, or social profiles, as well as through web3 methods like crypto wallets. Additionally, Privy supports embedded wallets, eliminating the need for users to have a self-custodial wallet prior to exploring your app. Privy is compatible with most EVM chains, including Base.