Build API reference: add mkdocstrings pages for all public classes and wire docs dep group

[Copilot]

The MkDocs site only had reference pages for Data and Stats, leaving Portfolio, build_data, Plots, and Reports undocumented. The docs toolchain also had no declared dependencies, relying entirely on ad-hoc uvx installs.

Changes

New reference pages (docs/reference/)

• api.md — build_data() function
• portfolio.md — Portfolio class
• plots.md — Plots class
• reports.md — Reports class

docs/mkdocs.yml — extended nav to surface all six reference pages; existing mkdocstrings config (Google-style, show_signature_annotations, members_order: source) renders per-method docs automatically from existing docstrings.

pyproject.toml — added docs dependency group (mkdocs, mkdocs-material, mkdocstrings[python]) so uv sync --all-groups in rhiza_book.yml installs them deterministically rather than relying solely on ephemeral uvx resolution.

No workflow changes required — rhiza_book.yml already calls make book → mkdocs-build.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Build API reference — Add mkdocs + mkdocstrings to generate per-function docs from existing docstrings; wire into rhiza_book.yml</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Build API reference — Add mkdocs + mkdocstrings to generate per-function docs from existing docstrings; wire into rhiza_book.yml #407

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