Add module-level docstring to pystore/__init__.py #103

Goal

The package's pystore/__init__.py currently has no top-level module docstring — there's only a brief """ temp fix for fastparquet 0.3.2 and numba 0.45.1""" inline comment near the legacy numba shim. Add a proper module docstring at the very top of the file (right after the license header, before any imports).

Acceptance criteria

• A module-level docstring is added as the first statement in pystore/__init__.py (immediately after the license comment block).
• The docstring describes what PyStore is in 1-2 sentences (flat-file datastore for pandas time-series data, built on Dask/fastparquet).
• Includes a minimal usage example (3-5 lines) showing `set_path`, `store()`, `collection()`, `write()`, `item()`.
• Mentions where to find more documentation (link to README on GitHub).
• No code/behavior changes — docstring only.

Out of scope

• Changes to any other file
• Changes to existing imports or __all__
• Updating the existing inline comment about the numba shim (leave it as-is)

Verification

• python -c "import pystore; print(pystore.__doc__)" should print the docstring
• Existing tests still pass: pytest tests/