Document CSNAKES_REDIST_CACHE environment variable with platform-specific examples

[Copilot]

The CSNAKES_REDIST_CACHE environment variable was already implemented in the RedistributableLocator class but was missing from the documentation. This PR adds comprehensive documentation for this environment variable across all relevant examples.

What this PR does

• Documents the CSNAKES_REDIST_CACHE environment variable in all files that contain FromRedistributable() examples
• Explains that it allows users to override the default cache folder location where Python redistributable packages are downloaded and stored
• Provides platform-specific examples with proper syntax highlighting for Windows (Command Prompt and PowerShell), Linux, and macOS
• Follows proper markdown code block conventions to avoid mixing shell syntaxes

Files updated

• docs/getting-started/installation.md - Main installation guide with detailed explanation and platform-specific examples
• docs/getting-started/first-example.md - First example tutorial with usage tip
• docs/user-guide/basic-usage.md - Basic usage guide with cache location note
• docs/user-guide/deployment.md - Deployment documentation with cache customization info
• docs/advanced/free-threading.md - Free-threading example with usage tip
• docs/advanced/troubleshooting.md - Troubleshooting guide with cache location tip
• docs/advanced/native-aot.md - Native AOT example with cache customization note
• docs/advanced/signal-handlers.md - Signal handlers example with usage tip

Example usage

Windows (Command Prompt):

set CSNAKES_REDIST_CACHE=C:\MyCustomCache

Windows (PowerShell):

$env:CSNAKES_REDIST_CACHE = "C:\MyCustomCache"

Linux/macOS:

export CSNAKES_REDIST_CACHE=/path/to/custom/cache

When set, Python redistributable files will be downloaded to the specified directory instead of the default application data folder (%APPDATA%\CSnakes on Windows, ~/.local/share/CSnakes on Linux, etc.).

Fixes #637.

---

💡 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.

[Copilot]

@tonybaloney 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.