docs: reworked README layout

[danielalyoshin]

Summary

Docs only. No code changes and no release.

• README layout. Centered header with badges (PyPI, Python, CI, license, Superset docs) and links to PyPI, the changelog, the Superset D1 docs and issues. Same style as the org profile README.
• Content kept. Every section and fact from the old README is still there. "Known limits" is now two bullet lists, and "Development" is split into setup, checks and integration tests.
• New parts, taken from the repo: a compatibility table for 0.2.x and 0.1.0 (from pyproject.toml and the changelog), a table for the parts of the connection string, the Superset menu path, and a "Contributing" section with what CI runs and which changes belong upstream.
• CHANGELOG. New [Unreleased] entry for the README change.

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Checked

• Rendered with GitHub's markdown API and with PyPI's readme_renderer. The centered header, badges, tables and notes show correctly in both.
• No mermaid or GitHub alert boxes, since PyPI shows those as raw text.
• Links to LICENSE and CHANGELOG.md are full URLs, so they also work on PyPI.

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Note

The PyPI page shows the README of the release, so it only changes with the next release.