Clean up and document analyses/ section

[dfsnow]

The analyses/ section is currently a bit of a mess. To clean it up we should:

• Update the README to explain its purpose (similar to the one under reports/
• Cleanup the existing workflow documentation
• Move the .qmd subsections to a dedicated directory
• Deduplicate the code in helpers.R, since a lot of it overlaps with R/helpers.R
• Move the "new feature" analysis doc to its own subdirectory, in case we add other types

In general, we want this subdirectory to contain the templates necessary to create one-off analyses (such as evaluating a new feature). The code (a .qmd file) and output (a .html file) should be saved in a subdirectory relative to the template. Something like:

.
└── analyses/
    ├── README.md
    └── new-feature/
        ├── template.qmd
        ├── modules/
        │   ├── aggregated_maps_categorical.qmd
        │   └── other_modules.qmd
        └── 0000-feature-name/
            ├── 0000-feature-name.qmd
            └── 0000-feature-name.html