Remove NuGet authentication with password option

[yordan-mitev]

• This PR removes the option to authenticate with a username and password in the Telerik NuGet server.
• Authentication with an API key is now the only officially supported option.

Authentication with a username and password will still work, but should not be advertised or mentioned in the docs due to security concerns.

[dimodi]

We do not use screenshots, because they are harder to maintain and usually not necessary. There are two exceptions - animated screenshots and screenshots with annotations. Other than that, we tend to remove existing screenshots when we update related content.

[yordan-mitev]

The screenshot was removed.

[yordan-mitev]

@dimodi , this is a quick summary of the changes since your last review:

• Move the API key download instructions to a template.
• Update the deployment/nuget-keys.md and installation/nuget.md files to use the instructions from the template.
• Remove the screenshot.
• Update the template for the Getting Started articles to reflect the requirement to use an API key instead of a password.