Updated documentation

[JulianTrommer]

Reworks the Optuna.jl documentation from a mostly API-reference page into a workflow-oriented docs site.

• Adds a home page and overview of core Optuna.jl concepts
• Adds a first optimization tutorial with study setup, suggestions, pruning, artifacts, and results
• Updates the polynomial fitting example into an end-to-end training-loop workflow
• Expands the API reference with explanatory sections before the docstring listings
• Updates the Documenter page structure

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[JulianTrommer]

@codex review

[chatgpt-codex-connector]

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[JoKircher]

Great work. Have not checked the examples. Will be done in a blocked issue that should be unblocked now!