Update documentation to remove `defaults` channel and suggest miniforge

[jashapiro]

As part of https://github.com/AlexsLemonade/OpenScPCA-analysis/issues/705, we will want to update all documentation to suggest using a miniforge installation rather than miniconda

A probably incomplete list of pages that need to be updated:

• [ ] README.md
• [ ] Conda setup
  • Change recommendation to miniforge with associated instruction updates (no GUI installer)
  • Remove conda config --add channels defaults as we don't want the GUI!
  • Add a note about what to do if you already have a conda installation?
• [ ] [Docker images](https://github.com/AlexsLemonade/OpenScPCA-analysis/blob/main/docs/ensuring-repro/docker/docker-images.md?plain=1
  • Change suggested conda base image
  • Change template for R/conda images to follow #711