docs: accurate package metadata; PyPI name no longer presented as decided (#58)

[Sean-Koval]

Refs #58 (the PyPI distribution name is still an owner decision, so this does not close it).

Re-verified against main

Most of #58 was already fixed by #70 and later PRs:

• crates/openquant/Cargo.toml: homepage is the real site, mlfinlab is gone from keywords, repository/readme/license/description are present.
• crates/pyopenquant/Cargo.toml: repository, readme, homepage and publish = false are present.
• pyproject.toml: accurate description, requires-python = ">=3.11" with 3.11/3.12/3.13 classifiers (CI tests 3.11 and 3.13), [project.urls] present, dependency bounds (numpy<3, polars<2, pyarrow<26) contain the locked versions (numpy 2.4.6, polars 1.44.2, pyarrow 25.0.1).
• README: title, what the library is, build-from-source install, docs link, CI/docs badges, 35% bench threshold (matches CI/justfile), no "production baseline" claim.
• grep -ri openquant-rs finds only .ai-dlc/work/rel-name-metadata.toml (the mirror of this issue's text).

Changed here

• crates/openquant/Cargo.toml: documentation → https://open-quant.github.io/openquant/api/rust/openquant/ (the rustdoc the docs site publishes), instead of the site root.
• pyproject.toml: Changelog URL added; a comment marks name = "pyopenquant" as a placeholder pending the Choose a distribution name and make package metadata and README accurate #58 decision (the name itself is unchanged).
• README, Overview, Quickstart, Python bindings setup, and the pyopenquant Cargo.toml comment stated "will be published as pyopenquant". They now say the import name is openquant and the PyPI name is not decided yet, keeping the warning not to pip install openquant.
• CHANGELOG entry; changelog page regenerated; last_authored bumped on the edited pages.

Every metadata/badge URL was checked with curl -L and returns 200: site root, /api/rust/, /api/rust/openquant/, /quickstart/, the repo, issues, CHANGELOG.md, both workflow badges, the shields.io license badge.

For the maintainer: GitHub settings (not changed by this PR)

The current description is generic and there are two topics (finance, machine-learning); the homepage is already set. To finish that item of #58:

gh repo edit Open-Quant/openquant \
  --description "Rust implementations of the methods in Advances in Financial Machine Learning, with Python bindings" \
  --homepage "https://open-quant.github.io/openquant/" \
  --add-topic rust --add-topic python --add-topic pyo3 --add-topic quant --add-topic afml

Left for #58

• Choose the PyPI distribution name, set it in pyproject.toml and drop the placeholder comment and the "not decided" sentences.

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