docs: add manual cache flush note to widget cache system

[carolhidalgos]

Summary

• Adds a note explaining that users can manually clear a widget's cache from Edit mode in the Admin Console (useful after changing widget config or a Data Analytics function)
• Adds a new screenshot (dashboard_clear_cache.png) illustrating the cache clear action

Test plan

• Verify the note renders correctly in the docs site
• Verify the new screenshot displays properly at the correct path

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