[Docs] Add a single exhaustive Environment-Variable reference table

[pcuci]

Env vars are scattered across docs/examples (LLM_, SANDBOX_, AGENT_, hidden debug flags). This fragmentation forces source-code grepping.

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[enyst]

I hear you, this is underdocumented for sure! We have this page on configuration:
https://docs.all-hands.dev/modules/usage/configuration-options#llm-configuration

It's focused on the toml, but all these vars are also available as env vars, and that's not obvious perhaps. It seems like maybe we could add a ToC at the top of the page, and a description and/or examples of the rule of formation for env vars.