Move Python generation step into Docker, no local Python required

Add python3 and python3-yaml to the Docker image so generate_cards.py
runs inside the container. Both the generate and build-blanko Makefile
targets now use docker run instead of a local python3 call.
Remove Python/PyYAML from the README prerequisites.

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

Dockerfile

@@ -5,6 +5,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     texlive-pictures \
     texlive-fonts-recommended \
     make \
+    python3 \
+    python3-yaml \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /workspace