docs: document docker_volumes config for shared host directories

The Docker backend already supports user-configured volume mounts via
docker_volumes, but it was undocumented — missing from DEFAULT_CONFIG,
cli.py defaults, and configuration docs.

Changes:
- hermes_cli/config.py: Add docker_volumes to DEFAULT_CONFIG with
  inline documentation and examples
- cli.py: Add docker_volumes to load_cli_config defaults
- configuration.md: Full Docker Volume Mounts section with YAML
  examples, use cases (providing files, receiving outputs, shared
  workspaces), and env var alternative

cli.py

@@ -158,6 +158,7 @@ def load_cli_config() -> Dict[str, Any]:
             "singularity_image": "docker://python:3.11",
             "modal_image": "python:3.11",
             "daytona_image": "nikolaik/python-nodejs:python3.11-nodejs20",
+            "docker_volumes": [],  # host:container volume mounts for Docker backend
         },
         "browser": {
             "inactivity_timeout": 120,  # Auto-cleanup inactive browser sessions after 2 min