Clarify installation steps when Node.js is already installed on Windows

Description

It would be helpful to make the installation documentation clearer for users who already have Node.js installed directly on Windows before installing NVM for Windows.

As a new user, it wasn't immediately clear whether the existing Node.js installation should be removed first or whether NVM can safely be installed alongside it.

Suggested Improvement

It would be useful to add a clearly visible section to the installation documentation explaining:

• Whether an existing Node.js installation should be uninstalled first
• Whether existing global npm packages are affected
• How PATH conflicts can be detected
• What users should do if node still points to the old installation
• How to verify that NVM is managing the active Node.js version

Example Verification

nvm version
nvm list
nvm use 22
node -v
where.exe node