Netlify CMS has been rebranded to Decap CMS

[itsmejoeeey]

Netlify have announced new maintainers for Netlify CMS. In the process the project has been rebranded to Decap CMS. https://www.netlify.com/blog/netlify-cms-to-become-decap-cms/

This repository should be updated to match the upstream.

• [x] Update repo name
• [x] Update docker images
• [x] Update package names
• [x] Update documentation