docs: explain configuration before creating custom environments

[shabeeth2]

Summary

Clarify that hatch env create does not define environments; it only creates environments that are already defined in project configuration.

This updates the command docstring to explain the expected configuration locations:

• pyproject.toml: [tool.hatch.envs.ENV_NAME]
• hatch.toml: [envs.ENV_NAME]

The implicit default environment remains available without an explicit definition.

Why

The previous docstring was ambiguous and could be read as though hatch env create were responsible for creating or modifying environment configuration. In practice, the command reads existing configuration and creates the environment from it.

Related issue

Related: #1504

Validation

• hatch run docs:build-check passed
• Reviewed the CLI help text and rendered environment documentation
• git diff --check passed

AI assistance: OpenAI Codex assisted with the edits, description and verification.