Improve vcs-versioning API: expose infer_version_string and enhance overrides

- Expose infer_version_string in main module for clean imports
- Remove references to non-existent read_named_env function
- Add comprehensive EnvReader documentation with examples
- Integrate logging configuration with GlobalOverrides context manager
- Simplify logging setup by using log_level() property directly
- Update integrator guide with corrected import patterns
- Fix get_version_for_build example to use dist_name parameter

All changes maintain backward compatibility. Tests passing: 42/42

Author: RonnyPfannschmidt