Change conda installation recommendation from miniconda to mambaforge

[adamltyson]

• conda is pretty slow in many relevant cases (e.g napari)
• conda-forge is likely the best default channel

mambaforge is more complex to install, and there's a lot to explain here for newcomers.

I suggest we address neuroinformatics-unit/HowTo#29 and then we have a clear resource to point to.

I think there's a lot to test here (e.g. multiple operating systems), so I suggest we stick with miniconda for the next course (in 2 weeks) and aim to migrate for the one after that.