Updated provider docs docs to include tip to use service credentials for NordVPN

[limxingzhi]

Breaking change

<placeholder>

Proposed change

NordVPN requires us to use server credentials instead of our regular login. Updated the docs to reflect that.

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality to this container)
• Breaking change (fix/feature causing existing functionality to break)

Additional information

• This PR provides more information about using server credentials.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated