docs: make core workflows verifiable

[stefan-jansen]

Closes #63

Summary

• Rewrites the landing page and quickstart around one offline, deterministic feature-computation workflow.
• Rebuilds the Book Guide against companion revision d2edec54b1c7a6a9d7a97d8129eb05db4491e1eb, with direct links and explicit relationship labels.
• Links all eight task guides to current notebook paths and distinguishes direct Engineer calls from manual or related workflows.
• Documents optional extras, calibration requirements, experimental surfaces, and unsupported scope.
• Adds fractional-differencing API reference coverage and removes obsolete public audit evidence.
• Makes the Book Guide table readable on narrow screens.

Capability inventory

Capability	Task guide	Exact API or runnable example	Book relationship
Feature computation	Features	compute_features, pipeline and complete-workflow examples	Direct calls in chapter and ETF notebooks
Feature discovery	Discovery	FeatureCatalog, executable docs block	Direct registry use in the ecosystem notebook
Labeling	Labeling	labeling API and example	Direct chapter call; ETF notebook marked related
Alternative bars	Alternative Bars	sampler API and example	Direct calls in three microstructure notebooks
Dataset building	Dataset Builder	builder API and complete-workflow example	No direct book call claimed
Preprocessing	Preprocessing	scaler API and examples	Direct StandardScaler call
Fractional differencing	Fractional Differencing	fdiff API and example	Direct chapter and ETF calls
ML readiness	ML Readiness	metadata and preprocessing APIs	Mixed direct and manual chapter workflow

Verification

• uv run ruff check src/ tests/ examples/ scripts/
• uv run ruff format --check src/ tests/ examples/ scripts/
• uv run ty check
• uv run pytest tests/ -q: 3940 passed, 57 deselected
• uv build
• uv run python -c "import ml4t.engineer"
• uv run mkdocs build --strict
• uv run python tests/documentation_workflows.py
• Six repository examples and all documentation workflows passed from a clean wheel installation.
• All 54 book-link occurrences resolve to 16 unique paths at the pinned companion revision.
• All 499 rendered internal documentation targets resolve.
• Desktop and 390 px layouts were inspected for the landing page, quickstart, Book Guide, and generated API reference.

Boundaries retained

• No checked notebook directly calls create_dataset_builder.
• The ETF labeling notebook is related workflow evidence, not a direct Engineer example.
• The Databento notebook needs vendor data and was semantically checked rather than executed.
• DuckDB storage remains experimental; transfer_entropy remains unsupported.
• This PR changes documentation and theme CSS only, so it does not require a package release for functional compatibility.

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