compose admin config from partials

## Use Case

I am finding the admin config fussy to edit the larger it gets, especially as the disease and normal cell line configs overlap, might be nice to split them up.

Proposal:
- split config into smaller `.yml` files: cell line, disease cell line, gene name, disease, pages, etc
- compose them into the larger config in build/CI step

Option B:
- I think `gatsby-plugin-netlify` could compose `.ts` configs (instead of `.yml`) which might have less syntax headaches than the raw yml
- downstream easier, but more set up/migration

## Acceptance Criteria

_Please describe how you know this is done_

## Details

_Please provide any helpful specifications_