Cheatsheets reorganized by topic

Currently, our cheatsheets are organized _by package_ rather than by a conceptual topic in scRNA-Seq analysis. We might want to re-organize these to be by topic instead.

To achieve this, we would have to determine:
- What are those topics?
- How many PDFs should be created?

Along the way, we would need to also update cheatsheet links and references throughout materials (`training-modules` and `training-specific-template`).

Start by inventorying the existing package-organized cheatsheets and their links and references throughout the training-modules and training-specific-template materials. Identify the conceptual scRNA-Seq topics and decide how many PDFs they require. Done means the cheatsheets are reorganized by topic and all affected links and references are updated.

Written by the indexing model from the issue text.