installation instructions suggest incorrect mamba installation

[orbeckst]

Describe the bug
https://userguide.mdanalysis.org/stable/installation.html#conda suggests to install mamba inside the mda env

conda create --name mdanalysis
conda activate mdanalysis
conda install -c conda-forge mamba

but this is not recommended in the official mamba instructions for mamba with existing conda where it is explicitly said

WARNING: Installing mamba into any other environment than base is not supported.

See also #361 (comment)

Expected behavior
Follow best practices.

Actual behavior
Wrong recommendation

[orbeckst]

I accidentally closed by moving it into "done" in the project board -- oops.

[orbeckst]

Note that the latest mamba docs don't have an entry for mamba with existing conda any more.