Add Section: how to downsample a large quantity of data

Suggested as this is a common recurring issue that users encounter. We should provide systematic guidance for this.

The issue names no file, test, or existing section; start by inspecting the repository's documentation structure and conventions. Establish the scope of the downsampling guidance and identify where this section belongs. Done means the repository contains a systematic section addressing how users can downsample large quantities of data.

Written by the indexing model from the issue text.