i tried adding https://www.netlifycms.org/docs/add-to-your-site/ to my personal site today. it failed. first i tried adding just admin/index.html and config.yml. this deployed but i wasnt able to navigate to /admin on the deployed site. so i suspected it was reactrouter/gatsby fouling it up. so referring to gatsby-starter-netlify-cms i thought i needed gatsby-plugin-netlify-cms. however that brought a bunhc of baggage i didnt need.
i will have to come back again to retry.