docs: replace browser history example with happy planet index dataset

[hydrosquall]

Motivation

• Provide a generally relevant, statistically interesting, and meaningful dataset for people to do interactive visualization with

Changes

• Check in + document a sample dataset using csvs-to-sqlite: https://github.com/simonw/csvs-to-sqlite
• Explain how to update it + why it was chosen

Testing

• Note you can set a custom rowcount using the _size URL parameter

  http://127.0.0.1:8001/happy_planet_index/hpi_cleaned?_size=137