Adds Grouping data (with hexbin, histogram) in Charts

[allisonhorst]

Addresses Mike's feedback in #522 :

To align with Plot’s mental model, perhaps better to combine this and the hexbin example into a bin.md that demonstrates group, bin, and hexbin transforms?

This update:

• Combines hexbin and histogram snippets into a single page, docs/charts/grouping-data.md
• Removes separate hexbin, histogram pages
• Updates observablehq.config.ts
• I did not move over two bar chart examples that use group transforms (Top 10 sorted, weighted sum), thinking that users would be more likely to look in the "Bar" page for those. Happy to move if they should also be in the grouping-data.md page.
• Current naming: "Grouping data" in sidebar, and as page title.

[Fil]

added a few small suggestions