feat: Reorganize environment variables documentation for clarity

[jancurn]

Summary

Restructured the environment variables documentation to improve organization and clarity by distinguishing between system and custom environment variables, and reorganizing sections for better logical flow.

Key Changes

• Reorganized introduction: Changed from "two ways to set up" to "two sources" of environment variables (system vs. custom), with clearer distinction between them
• New "System environment variables" section: Added prominent info box clarifying that system variables are run-time only and not available during build, even with the build process option enabled
• New "Custom environment variables" section: Grouped the two setup methods (actor.json and Apify Console) under a parent section with shared precedence rules
• Restructured subsections: Converted "Set up environment variables in..." sections to subsections (###) under the new Custom environment variables parent
• Reordered content: Moved "Use the Configuration class" section before "Access environment variables" for better logical flow (convenience method before raw access)
• Enhanced "Build-time environment variables" section:
  • Clarified that build variables apply only to builds, not runs
  • Added explicit instructions to enable the build process option
  • Added caution box explaining that only user-defined variables (not system variables) are passed to builds
  • Moved the "Variables set during the build" note to the end with additional context about build freezing
• Minor formatting: Changed "Environment Variable" to "Environment variable" in table header for consistency

Notable Details

The changes maintain all existing content while improving the document structure to make it clearer that:

1. System variables are automatically provided by Apify and are run-time only
2. Custom variables are user-defined and can optionally be passed to builds
3. The Configuration class is the recommended approach for accessing configuration
4. Raw environment variable access is still available as an alternative

https://claude.ai/code/session_0121d3Sgy1UxMnpVZzBEQCNA