build-lectures: no `_build/.doctrees` clear (ipywidget mimetype rendering)

[mmcky]

Surfaced by the v0.8.0 validation study (composite vs. native pipelines).

Problem

The native lecture-python-intro / lecture-python-programming pipelines run rm -r _build/.doctrees before the final HTML build to fix ipywidget mimetype rendering. The composite build-lectures has no equivalent — there are no .doctrees references anywhere in the repo.

Why it matters

ipywidget-heavy lectures may render differently (mimetype issues) when built via the composite than via the native pipeline they migrated from.

Suggested fix

• Spot-check ipywidget-heavy lectures built through the composite against the native output.
• If the divergence reproduces, add an optional pre-final-build _build/.doctrees clear (behind an input) to the build-lectures HTML path.

Context: QuantEcon/lectures#11.