Add GitHub Copilot instructions for FinOps toolkit

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file to provide context and guidelines for the GitHub Copilot SWE agent when working on the FinOps toolkit repository.

What's included

The instructions file covers:

• Repository overview - Context about the FinOps toolkit's purpose and structure
• Core principles - Quality standards, Microsoft style guide adherence, and zero-lint policies
• Development guidelines - Best practices for features, bug fixes, and documentation updates
• Language-specific guidance - PowerShell, Bicep, and other technology-specific conventions
• Common pitfalls - What to avoid when making changes
• Key resources - Links to FinOps Framework, Microsoft docs, and internal guidelines

Key features

• Emphasizes the repository's focus on cost optimization and FinOps best practices
• Incorporates Microsoft style guide principles (sentence casing, brevity, clarity)
• References existing documentation in docs-wiki/ folder
• Follows the branching strategy and development process
• Includes markdown linting compliance with appropriate rule disables

The instructions are designed to help Copilot make more context-aware suggestions that align with the project's standards and goals.

Fixes #1681.

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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.