Document support for variables in environment spec

[pavithraes]

Context

In conda-incubator/conda-store-ui#354, we add support for passing environment variables in the conda-store-ui through the YAML editor.
This allows users to create envs for GPU-powerd workflows, by adding the CONDA_OVERRIDE_CUDA.
This needs to be documented in https://conda.store/conda-store-ui/tutorials/create-envs#graphical-interface, with a new 3rd level heading after the pip section.

Value and/or benefit

Documenting available features for conda-store users.

Anything else?

No response

[nkaretnikov]

Status update: the respective documenation PR is in review, but it's blocked until conda-incubator/conda-store-ui#354 lands.