Add instructions on how to install Mamba

[peanutfun]

The Mamba installation instructions are confusing. We should add instructions how to properly install it for newcomers. In the long term, we might switch to Conda again, which is easier to install.

[bguillod]

Since conda now uses mamba as the engine, wouldn't it be easiest to just switch back to conda?

[peanutfun]

Unfortunately not. This is a short-term solution for easier installation because we are stuck in the transition to Conda, see #913. It seems like Conda works fine, but Anaconda does not. We would ideally like to support both. Until this is resolved, we will stick to Mamba.