Update README.md for clarity and template availability

[shibayan]

This pull request updates the README.md to clarify and modernize documentation for Dev Container Templates focused on Azure Functions development. The changes reorganize template information, improve usability instructions, and make the supported languages and versions more prominent.

Documentation improvements:

• Replaced the previous template descriptions with a clear, concise table listing available Azure Functions templates, their languages/runtimes, and supported versions.
• Added instructions for using templates with VS Code and the Dev Container CLI, making it easier for users to get started.

Usability and clarity:

• Updated introductory and section headings for consistency and clarity, emphasizing the multi-container environment and Azurite storage emulator included with each template.
• Noted that all templates include Azure Functions Core Tools, with selectable versions (default: latest).

[Copilot]

Pull request overview

This pull request modernizes and improves the main README.md documentation for the Dev Container Templates repository. The changes reorganize template information into a clear table format, add practical usage instructions, and better highlight the multi-container architecture with Azurite storage emulator support.

Changes:

• Restructured template information into a table showing languages/runtimes and supported versions
• Added comprehensive usage instructions for both VS Code UI and Dev Container CLI
• Enhanced introductory description to emphasize the multi-container environment and included tools

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