Add new "Features" docs section and Preview & Load video (#2887)

apps/zui/docs/advanced/README.md

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 # Advanced Guides

apps/zui/docs/developer/README.md

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 # Developer Resources

apps/zui/docs/features/Preview-Load.md

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+
+# Preview & Load
+
+The video below shows how you can preview and
+[shape](https://zed.brimdata.io/docs/language/shaping) your data as you load it
+into a [Zed lake](https://zed.brimdata.io/docs/commands/zed) with Zui. It also
+touches on the significance of setting a
+[pool key](https://zed.brimdata.io/docs/commands/zed#pool-key) and customizing
+the stacked bar chart.
+
+To further explore the Zed concepts shown in the video, you may want to read
+the [`zq` tutorial](https://zed.brimdata.io/docs/tutorials/zq) and use the
+[`prs.json`](https://github.com/brimdata/zed/raw/main/docs/tutorials/prs.json)
+test data.
+
+<iframe width="560" height="315" src="https://www.youtube.com/embed/ywPG1a9FLX0?si=xykqAgIPpsocYhJz" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>

apps/zui/docs/features/README.md

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+
+# Features
+
+Learn how to get the most from Zui features. Topics described here include:
+
+* [Preview and load data](Preview-Load.md)