Clean up packaging metadata and build config

[zonca]

This PR cleans up a set of packaging issues that showed up during release prep and artifact verification. Each fix is isolated in its own commit for easier review.

Included changes:

• remove the stale Python 3.8 classifier to match requires-python >=3.9
• remove the dead importlib_metadata conditional dependency
• update the release instructions to install .[test] before running pytest
• remove stale Python pandoc package entries from docs dependencies
• preserve the src/ layout in sdists so hatch-vcs can write src/pysm3/_version.py
• remove the redundant wheel force-include that broke wheel builds from sdists
• fix README RST markup so the long description renders on PyPI

Verification:

• python -m build
• python -m twine check dist/*