Fix RTD docs build failure caused by Rust/maturin compilation timeout

[igerber]

Summary

• Remove Rust toolchain and pip install . (maturin build) from .readthedocs.yaml — the Rust backend is unnecessary for docs and exceeds RTD build limits
• Install only runtime + docs dependencies via post_install hook
• Add sys.path.insert in docs/conf.py so autodoc imports diff_diff from the checked-out source (preserving the PR SyntheticDiD docs/examples use zeta_omega/zeta_lambda, but diff-diff==2.3.0 constructor only accepts lambda_reg/zeta #146 fix for version-matched docs)

Methodology references (required if estimator / math changes)

• N/A — no methodology changes

Validation

• Tests added/updated: No test changes (build configuration only)
• Local verification: cd docs && python -c "import sys; sys.path.insert(0, '..'); import diff_diff; print(diff_diff.__version__)" prints 2.3.1
• RTD build should be triggered by this PR for end-to-end validation

Security / privacy

• Confirm no secrets/PII in this PR: Yes

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Generated with Claude Code

[github-actions]

Overall assessment: Looks good

Executive summary

• No estimator, inference, or identification logic changes in the diff; Methodology Registry not implicated.
• RTD now skips the Rust build and installs runtime + docs deps explicitly, with sys.path adjusted to import from source.
• Maintainability risk: docs dependency list is duplicated in .readthedocs.yaml and can drift from pyproject.toml.

Methodology

• Severity: P3. Impact: No methodology/estimator/SE changes in this PR; registry cross-check not required. Fix: None.

Code Quality

• Severity: P3. Impact: No production code changes beyond docs config and path setup. Fix: None.

Performance

• Severity: P3. Impact: No runtime performance changes. Fix: None.

Maintainability

• Severity: P2. Impact: Dependencies are duplicated in .readthedocs.yaml and pyproject.toml, risking drift that can break RTD builds or omit docs deps. Fix: Introduce a single source of truth (e.g., docs/requirements.txt generated from pyproject.toml and install via pip install -r docs/requirements.txt). Refs: .readthedocs.yaml:L10-L19, pyproject.toml:L32-L58.

Tech Debt

• Severity: P3. Impact: No new tech debt beyond config change. Fix: None.

Security

• Severity: P3. Impact: No secrets or security-relevant changes in the diff. Fix: None.

Documentation/Tests

• Severity: P3. Impact: No automated validation added for the new RTD install path; regressions may surface only on RTD. Fix: Consider a lightweight CI smoke test (e.g., python -c "import sys; sys.path.insert(0, '..'); import diff_diff" from docs/) to catch import failures early. Refs: docs/conf.py:L9-L16.