ci: use dev-docs container for docs CI (#445)

* ci: use dev-docs container for docs CI

Run docs workflow inside ghcr.io/wphillipmoore/dev-docs:latest, removing
the custom Python setup action. uv and Python are pre-installed in the
container; uv sync --group docs still runs for mkdocstrings dependencies.

Relates-to: standard-tooling-docker#25

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* chore: re-trigger CI with updated PR body

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: wphillipmoore