Add VS Code for the Web badge to README

[Copilot]

Adds the "Open in VS Code for the Web" badge alongside existing GitHub Codespaces and Dev Containers badges.

Changes:

• Added badge linking to vscode.dev/azure?azdTemplateUrl=https://github.com/azure-samples/azure-search-openai-demo
• Uses purple color and Visual Studio Code logo for brand consistency
• URL encoding uses + for spaces, matching existing badge conventions

The badge appears on line 28 of README.md:

[![Open in VS Code for the Web](https://img.shields.io/static/v1?style=for-the-badge&label=VS+Code+for+the+Web&message=Open&color=purple&logo=visualstudiocode)](https://vscode.dev/azure?azdTemplateUrl=https://github.com/azure-samples/azure-search-openai-demo)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add Open in VS Code for the Web Badge to the README</issue_title>
<issue_description>Add the badge for "Open in VS Code for the Web" next to the Dev Containers and other badges in the readme. Here is the Badge -

</issue_description>

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

• Fixes Add Open in VS Code for the Web Badge to the README #2870

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[pamelafox]

@spboyer Did you verify this repo works in VS Code for the Web?