Improve connecting via EIP-6963 content

[alexandratran]

Now that EIP-6963 is the recommended method of connecting to MetaMask, and we also want to deprioritize the SDK content, some of our documentation structure and code samples are outdated. This issue is to:

• Remove the Quickstart section
• Update the Detect wallet section:
  • Rename it to "Connect to MetaMask"
  • Recommend EIP-6963 as the primary method for connecting, whether directly or via third-party library
  • Add EIP-6963 code examples for vanilla JS
  • We no longer recommend detecting MetaMask via @metamask/detect-provider (should this content be removed completely?)
• Audit and update code samples throughout the docs that use window.ethereum
• Remove Access a user's MetaMask provider

Other places that may require related updates:

• Ethereum provider API concept and reference
• Wallet interoperability
• Detect wallet when connecting to a Snap

[alexandratran]

Fixed by #1214