Update Getting Started optional tools section with latest Node14 Docker images

https://github.com/AdobeDocs/project-firefly/blob/main/src/pages/getting_started/index.md

1) Node10 and Node12 images are mentioned, but Node14 images should be preferably used.
2) The documentation could be clearer about when to download the images manually (e.g. on slow connections).

Start with src/pages/getting_started/index.md and read the optional tools section. Update the documented Node image guidance to prefer Node14 and clarify when users should download images manually, such as on slow connections. Done means the Getting Started page reflects both points accurately.

Written by the indexing model from the issue text.