Add theming docs for Plotly, Altair, and Vega-Lite

[snehankekre]

📚 Context

Streamlit 1.16.0 introduces a new Streamlit theme for Altair, Vega lite, and Plotly charts.

🧠 Description of Changes

• Adds a section on Theming to the st.altair_chart, st.plotly_chart, and st.vega_lite_chart commands' API reference pages.
  • The section includes a description of what theming is, how to turn it on/off, what the default behavior is, and how Streamlit accounts for customizations of colors and chart configs.
• Embeds apps in all three API reference pages demonstrating the use of the theme argument, with and without custom chart configs.
• Updates the API reference images for all three commands.

💥 Impact

Size:

• Small
• Not small

🕵️ For reviewers

PTAL at the API reference changes for:
Altair | Notion
Plotly | Notion
Vega-Lite | Notion

🌐 References

• Notion

Contribution License Agreement

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