DOC: Convert economic_curves into a placeholder linking to Open-Source Modelling

[fumitoh]

Closes #109.

The economic_curves library was a mirror of insurance_python by Open-Source Modelling (formerly Actuarial Algorithms). The upstream repositories have kept evolving while the mirror in lifelib was left untouched, so the shipped scripts and the rendered notebooks had become stale.

This PR removes the mirrored contents and points to the upstream repositories instead.

Changes

lifelib/libraries/economic_curves/ — all 27 files (scripts, notebooks, LICENSEs, xlsx) removed, replaced by a single README.md explaining that the library is now only a placeholder and where its contents went.

doc/source/libraries/economic_curves/index.md — rewritten as an introduction to Open-Source Modelling:

• what happened to the library and why;
• who Open-Source Modelling are, including the rebrand from Actuarial Algorithms;
• a table of the 12 algorithm folders in insurance_python, each linked, with the upstream source references. This is more than the 9 that were mirrored — Vasicek one/two factor, Hull-White one factor, Dothan one factor and singular spectrum analysis have been added upstream since;
• a table of the 5 notebooks in insurance_jupyter;
• a table of their other active repositories (Open_Source_Economic_Model, Light_Economic_Generator, Validation_Economic_Stochastic_Generator, EIOPA_all_curves, SFCR_using_Mistral_2025, IMF_GDP_pipeline, EDGAR_pipeline, insurance_matlab).

The 14 per-algorithm .md pages and .nblink files are deleted — they included READMEs and notebooks that no longer exist, so the build would break otherwise.

Reference plumbing — the economic_curves section is removed from the Notebooks page and from the front-page Notebooks card, but the notebooks_economic_curves label now lives on the new library page so the v0.8.0 release notes still resolve without rewriting them. The {module} economic_curves directive is kept for the same reason (:mod: references in updates.rst and the release notes).

setup.py — md added to the packaged extensions, so the placeholder README ships in the wheel and lifelib.create("economic_curves", ...) still works instead of raising KeyError. This also picks up one pre-existing file, smithwilson/smith-wilson-py/README.md.

A note on where the notebooks live

The issue mentions linking the notebooks in insurance_python, but that repository no longer contains notebooks — they have been split out into insurance_jupyter. The notebook links therefore point at insurance_jupyter rather than at paths that no longer exist.

Verification

• Built the new page in an isolated Sphinx run: all three tables render, every reference-style link resolves (43 external links, no unresolved literals), and both :ref:notebooks_economic_curves and `:mod:`~economic_curves resolve to the new page.
• All upstream URLs checked against GitHub, including the percent-encoded notebook filenames.
• lifelib.create("economic_curves", path) produces a folder containing just README.md.
• A full doc build was not run — the toolchain (nbsphinx, sphinx-gallery, pydata theme, modelx) is not installed in this environment.

No release-notes entry or updates.rst item was added, since there is no in-progress release file; that seemed like a release-time call.

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[fumitoh]

@open-source-modelling this takes the "just leave a link" option from #109 — your work is no longer mirrored here, and the lifelib page now introduces Open-Source Modelling and links straight to your repositories.

A few points where your input would be most useful:

1. How you are described. The page says Open-Source Modelling publishes "open-source algorithms for actuaries and risk managers", was formerly known as Actuarial Algorithms, and is developed and maintained by Gregor Fabjan and Qnity Consultants. Please correct anything that is off.
2. Notebook links. The issue mentioned the notebooks in insurance_python, but they now live in insurance_jupyter, so that is where the links point. Let me know if you would rather they point elsewhere.
3. Which repositories to feature. Besides insurance_python and insurance_jupyter, the page lists Open_Source_Economic_Model, Light_Economic_Generator, Validation_Economic_Stochastic_Generator, EIOPA_all_curves, SFCR_using_Mistral_2025, IMF_GDP_pipeline, EDGAR_pipeline and insurance_matlab. Happy to add, drop or reorder these — you know best which ones you want people sent to.

You mentioned a deeper integration as the other option. Nothing here blocks that later; the placeholder page is easy to replace if you would like to revisit it.

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