Add environment variables documentation

[KrishnaKapoor612]

Adds documentation for Metaflow environment variables.

This page explains:

• The METAFLOW_<CONFIG_NAME> override pattern used by configuration values defined in metaflow_config.py
• Commonly used environment variables grouped by category (User & Runtime, Metadata Service, Datastore, AWS Batch, Kubernetes, Debugging)
• The precedence order between environment variables, configuration files, and internal defaults
• Troubleshooting tips for common issues (e.g., "unknown user")

Note:
Some runtime-specific variables (e.g., METAFLOW_RUNTIME_NAME, METAFLOW_PRODUCTION_TOKEN) are read directly from the environment and are not defined via from_conf() in metaflow_config.py.

This PR targets the metaflow-docs repository as suggested in the original issue discussion.

[vercel]

@kKAPS22 is attempting to deploy a commit to the Metaflow Team on Vercel.

A member of the Team first needs to authorize it.

[KrishnaKapoor612]

Looks like the Vercel check requires org authorization.
Since this is a docs-only change, please let me know if this needs any action from my side.