Fix docs workflow: install mkdocstrings via uv

[taddyb]

Summary

• Switch the docs workflow from pip install zensical to uv sync --extra docs so the mkdocstrings[python] optional dependency is actually installed
• Run zensical build --clean via uv run --directory extractrs-python so it picks up the synced env

The previous workflow only installed zensical itself, which caused zensical build to fail with: mkdocstrings plugin is enabled, but mkdocstrings is not installed.

Test plan

• Workflow runs to completion on push to master
• site/ artifact is built and uploaded
• Pages deployment succeeds
• Rendered site shows mkdocstrings-generated accessor docs

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