[playwright vscode extension] Add getting started instructions to README

[Copilot]

The extension README was missing setup instructions for users, specifically how to start the browser tunnel and key configuration requirements.

Changes

• Getting Started section: Step-by-step instructions for starting/stopping the tunnel via Command Palette
• Requirements section: Documents terminal.integrated.shellIntegration.enabled must be enabled
• Auto-starting section: Shows workspace settings configuration with security note

Example workspace config

{
  "playwright-local-browser-server.autoStart": true
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>[playwright-local-browser-server] Provide instructions for starting browser server</issue_title>
<issue_description>In ./vscode-extensions/playwright-on-codespaces-vscode-extension/README.md, we do not instruct users to start the browser tunnel first.

• Write a blurb that instructions users to start the browser tunnel (and how to do so from the vscode command palette)
• Mention that this extension requires that terminal.integrated.shellIntegration.enabled setting must be true for this extension to work
• Also mention that developers can set autostart: true in their .vscode workspace settings file to true to have autostart automatically.
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [playwright-local-browser-server] Provide instructions for starting browser server #5591

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.