How to transit tools.get_env to Conan 2.x schema?

The current [migration guide](https://docs.conan.io/en/latest/conan_v2.html) covers most of important parts, but the migration is not totally clear about environment variables.

Some recipes, they collect external environment variables, and so far [tools.get_env](https://docs.conan.io/en/latest/reference/tools.html#tools-get-env) it's the current option.

With the introduction of [conan.tools.env](https://docs.conan.io/en/latest/reference/conanfile/tools/env.html), we should consider configuration files instead, but it's not totally clear how to do it.

It would be great if it was documented and specified as a replacement for `tools.get_env`.