chore(docs): tips for installing CN fonts (#14189)

### What problem does this PR solve?
Add tips for installing Chinse fonts under code sandbox. Otherwise,
`matplotlib `won't render Chinese correctly.

<img width="2082" height="1186" alt="sales_analysis"
src="https://github.com/user-attachments/assets/57e675ab-1e92-4662-9aeb-ad72a6121eb5"
/>



### Type of change

- [x] Documentation Update

agent/sandbox/sandbox_base_image/python/Dockerfile

@@ -4,8 +4,10 @@ COPY --from=ghcr.io/astral-sh/uv:0.7.5 /uv /uvx /bin/
 ENV UV_INDEX_URL=https://pypi.tuna.tsinghua.edu.cn/simple
 ENV MPLBACKEND=Agg
 ENV MPLCONFIGDIR=/tmp/matplotlib
+ENV MATPLOTLIBRC=/usr/local/etc/matplotlibrc
 
 COPY requirements.txt .
+COPY matplotlibrc /usr/local/etc/matplotlibrc
 
 RUN grep -rl 'deb.debian.org' /etc/apt/ | xargs sed -i 's|http[s]*://deb.debian.org|https://mirrors.tuna.tsinghua.edu.cn|g' && \
     apt-get update && \