Add full support for powershell in conda docs

[traversaro]

• Modify powershell section in https://github.com/robotology/robotology-superbuild/pull/885/files to specify that also mamba does not work, and so in general suggest ot stick to Command prompt
• Fix mamba use in powershell ( mamba init add powershell support mamba-org/mamba#1088 )
• Fix powershell in Gazebo ( On Windows Gazebo does not start in Powershell or Git Bash conda-forge/gazebo-feedstock#42 )
• (Optional) Fix powershell in ROS ( ROS tools not found in PowerShell on Windows RoboStack/ros-noetic#21 )

[traversaro]

Hopefully better docs should be provided in #891 .