doc: separate Miniconda installation and conda initialization

- Move the documentation for the conda configuration under "Porting"
- Move the instructions for initializing the user's shell for use with conda
  to a speparate section
- Add a new section for manually initializing the user's shell if they do not
  want to modify their startup files
- Emphasize which steps are "first-time setup" steps
- Add details about how the conda environment is activated during build/run