Fix issue with not building from sdist

[hajimes]

This PR fixes the issue reported in #90 that the package could not be built from the source distribution (sdist). It also adds tests to verify building from the source distribution, ensuring this issue does not recur.

The PR also includes a few minor documentation fixes and enhancements.

[hajimes]

To be clear, in mmh3 4.* this issue should not occur, as it was introduced in #79.

The commit #79 removed MANIFEST.in, because setup.py is no longer required by setuptools. On my local environment, the default setuptools configuration in pyproject.toml included *.h files during python -m build --sdist, but on other systems, this default configuration does not include them. I’m not sure why this discrepancy occurred, but in any case this PR resolves the issue by explicitly declaring *.h as additional package data."

[tool.setuptools]
include-package-data = true
# ...

[tool.setuptools.package-data]
mmh3 = ["*.h"]