docs(config): fix fallback provider config paths

[LeonSGP43]

Fixes #19691

Summary

• update the environment variable reference to document top-level fallback_providers as the current fallback chain shape
• clarify that fallback_model is still legacy/backward-compatible, not the recommended config
• fix the CLI reference so hermes fallback persists to top-level fallback_providers, not under model

Verification

• rg -n 'fallback_model|fallback_providers|under model|under model' website/docs/reference/environment-variables.md website/docs/reference/cli-commands.md hermes_cli/fallback_cmd.py gateway/run.py\n- git diff --check

[alt-glitch]

Likely duplicate of #19790 — same docs fix for stale fallback_providers config paths, both targeting #19691.

[alt-glitch]

Likely duplicate of #19790.