feat(block-kit): add data visualization guidance

[wgminer]

Summary

Extends the data-display guidance introduced in #167 with a focused set of recommendations for data_visualization charts:

• choose charts only when the shape of the data matters
• keep series and categories readable for the card size
• use color to distinguish series, not to communicate status or meaning
• choose bar, line, area, and pie charts according to their data shape
• handle long ranges, missing values, and empty data explicitly
• state the takeaway and important values in text

This intentionally leaves the fuller component anatomy, palette, sizing tables, and platform details in the source guidance rather than adding all of them to the skill.

Merging this PR adds the commit to the block-kit-design-references branch and updates #167.

Validation

• make lint
• make test-unit (21 passed)

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Latest commit: ed15928

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[salesforce-cla]

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