feat(cli): add --docstring-style flag to override config

[Alberto-Codes]

Problem

Projects using Sphinx/RST docstrings must set docstring-style = "sphinx" in [tool.docvet] to avoid false positives. If the project has no pyproject.toml or the user wants a quick one-off run (e.g., evaluating docvet on a new codebase), there's no way to set the style without editing config files.

Discovered while running docvet against boto3 — had to append a [tool.docvet] section to their pyproject.toml just to set the style.

Proposal

Add --docstring-style as a top-level CLI option (alongside --format, --output, --config):

docvet --docstring-style sphinx check --all

• Values: google (default), sphinx
• CLI flag overrides pyproject.toml value when both are set
• Available on the app callback so it applies to all subcommands

Context

• docstring-style config key shipped in Epic 34 (Story 34.1)
• Other top-level options (--format, --output, --verbose, --quiet, --summary, --config) already follow this pattern
• Zero new dependencies — just a typer option wired to the existing config field