Add a deprecation overlay that has a call-to-action button to our new book

[ThijsFeryn]

https://book.varnish-software.com/4.0/ and https://book.varnish-software.com/3.0/ are heavily outdated.

I'd like to have an modal-style overlay that greys out the content and displays a disclaimer text.

The modal has a button that links to the new book and the close icon allows users to still view the outdated content.

Here's the text:

The content of this book is heavily outdated and refers to a version of Varnish that is end-of-life.  We have published a new book that focuses on Varnish 6. 

Click here to download "Varnish 6 by Example".