Add alt text to images & charts for improved accessibility

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Summary

Add a way for developers to add alt text to commands such as st.image and chart objects.

Why?

For accessibility reasons, images, charts, and other visualizations should always have alt text that describes the content.

How?

This should probably be a parameter that the developer can set since we can't automatically generate alt text (even though this might be a cool LLM application ). Maybe just a parameter alt or alt_text or alternative_text that accepts a string.

Elements for which we could/should add this:

• st.image
• st.dataframe and st.table (see this article on accessible tables)
• all chart elements

Note that some of these elements also have other parameters to describe the content (e.g. st.image has a caption parameter), so we'd have to see how these two work together.

Additional Context

No response