docs: rewrite the README and fix the crashing quickstart

[Par-python]

Rewrites the README in the style of laya and fixes a bug: the first example anyone ran crashed.

Bug fixed

The Quickstart (README and docs/quickstart.md) ran shannon.rolling(s, window=20) on an 8-point series and raised ValueError: window (20) is larger than series length (8). The other three README examples used undefined variables (s, windows, labels) or a matcha_trends.csv that doesn't exist.

New layout

• Centered banner (light and dark), drawn from entroscope's own rolling spectral entropy by docs/assets/make_banner.py, which is adapted from the original banner design. The badges now include the docs site.
• Installation with an extras table.
• Quickstart on a noise → cycle series, with real outputs as # -> comments. It ends on the case for nine measures: Shannon entropy can't tell noise from the cycle (3.08 vs 3.23 bits), while spectral entropy can (6.05 vs 0.88).
• Why trust the numbers: the antropy / EntropyHub / scipy validation table.
• Sections on rolling entropy over time (including missing values), transfer entropy and divergence, and scikit-learn / polars.
• The nine measures: what each one captures and when to reach for it.
• Honest limits: where entropy lost to a simple rule (the training-divergence study), measured rolling speed (sample entropy ~3 s per 3,000 points), sample entropy's finite ceiling, the spectral definition difference, and transfer entropy's data needs.

Guarding it

tests/test_readme.py runs every Python block in README.md and docs/quickstart.md in order, and checks each numeric # -> value claim (14 in total) against what the line returns.

Notes

• The banner images use absolute raw.githubusercontent.com/.../master/... URLs so they also render on PyPI. They'll show as broken in this PR's preview until it merges.
• PyPI only shows the new README after the next release.

Test plan

• pytest: 250 passed, coverage 98.7% (includes the 2 new README tests)
• mkdocs build --strict
• Banner checked in both themes
• ruff check / ruff format --check with local ruff 0.15.15. CI runs the pinned 0.16.7 (pypi.org was unreachable from my machine).
• CI green

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