Disclaimer & direct link to Material theme removed

[devzeth]

The material theme was taken down from the Microsoft marketplace for potential malicious code. The author has renamed the repository and uploaded it again under a different name.

In order to remove any forward links to any potential malicious extension, we don't know what the original author further will do, I removed the direct link but kept the text as is.

The Imgur image was also removed as it no longer worked.

Added a disclaimer to clarify how the color theme was implemented in zed, and that the extension is not affected at all by any changes in Material theme, as we don't use its source code.