Run generated documentation examples in docs CI

[qciaran]

Summary

This draft first wires docs CI to run the same generated documentation example test path developers use locally:

• build docs dependencies with just python-ci-build-docs
• run generated docs examples with just docs-test
• keep mkdocs build after the executable docs check

The first commit intentionally does not fix the examples yet, so CI should expose the currently hidden failures in generated docs tests.

Expected failures

Locally, enabling this catches stale/broken examples in:

• docs/user-guide/simulators.md: examples use legacy simulator classes in .quantum(...) instead of current builder functions like state_vector(), sparse_stab(), etc.
• docs/user-guide/qec-guppy.md: the color-code runtime example stalls in HUGR execution.

Follow-up

I have local fixes prepared for the docs examples. With those fixes applied, just docs-test passes locally:

237 passed, 10 skipped, 5 deselected, 2 warnings