fix: expose markdown source as __file__

[tgolob]

Summary

• expose the Markdown or Python source path to code fences through __file__
• add a regression test that reads a neighboring fixture
• document the source-path global

Closes #52.

Validation

• uv run --group dev pytest -q — 44 passed
• uv run --group dev ruff check src tests/plugin_test.py
• uv run --group dev ruff format --check src tests/plugin_test.py
• git diff --check

The repository's existing mypy run still reports the pre-existing missing annotation for _registered_runners in src/pytest_markdown_docs/_runners.py.