Fix Sphinx roles in README that fail twine check (and gate it in PR CI)

[banesullivan]

The v0.4.0 release failed at the PyPI publish step because the README I added in #77 used Sphinx interpreted-text roles (:class:\pyvista.PolyData`, :class:`pyacvd.Clustering`) which twine check` rejects. Failure: https://github.com/pyvista/pyacvd/actions/runs/25574339938/job/75079011573

PyPI's RST renderer is plain docutils and doesn't know about Sphinx roles, so the long_description fails markup validation and the upload aborts. My mistake.

Fix

Replace the two :class:\...`references with plain``...``` literals. The accessor docstrings still use proper Sphinx roles since they're rendered by the API docs build, not by PyPI.

Prevent recurrence

Add twine check --strict dist/*.tar.gz to the build_sdist job in .github/workflows/build-and-deploy.yml so every PR exercises the same check the release job runs. --strict treats warnings as errors. The release path itself was already guarded; the gap was that PRs never built the sdist's metadata in a way that surfaced the failure before tagging.

Verification

Built the sdist locally and ran twine check --strict against it: passes. After this PR merges, the v0.4.0 release will need to be retagged (e.g. v0.4.1) so the publish step runs against the corrected sdist.