Document environment variables for conda configuration

[Adi-123456789]

Description

This pull request adds detailed documentation for configuring Conda using environment variables, addressing the lack of official guidance on this feature.

Key additions:

• Explanation of the standard naming convention (CONDA_<KEY>).
• How environment variables override configuration files and command-line options.
• A categorized list of common environment variables with usage examples (e.g., CONDA_SAFETY_CHECKS, CONDA_SSL_VERIFY, CONDA_PKGS_DIRS).
• Notes on handling special values such as booleans and lists.

This documentation aims to help both contributors and users understand and effectively use environment variables in Conda.

Related issue:

Fixes conda/conda#15300

[conda-bot]

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[Adi-123456789]

Hello! I’ve completed signing the Contributor License Agreement (CLA). Kindly refresh the CLA check at your convenience.
Thank you!

[travishathaway]

Hi @Adi-123456789,

Thanks for the submitting this pull request. Unfortunately, this is not the correct repository for it. I've transferred this original issue to the correct repository: https://github.com/conda/conda

I think this content would be a great addition to that document site though. Here's how I think it should be done:

1. Create a new page in this folder called, using_environment_variables.rst
2. Put the content from this pull request on that page
3. Add an entry for it on this page

If you need any help with this after opening the new pull request, please ping me. I'll be happy to help 😄.

Thanks again for taking the time to create this contribution!