Add prominent https://webkit.org/ link to top page

A developer/reader coming in to the https://github.com/WebKit/WebKit/
repo would likely find it pretty useful to also browse directly to the
top of the https://webkit.org/ site and explore all the other
information about WebKit which that site provides.

But the existing Readme at https://github.com/WebKit/WebKit/ doesn’t
include any link directly to the top of the https://webkit.org/ site.

Among the specific things linked to on the https://webkit.org/ site,
developers are likely to find the Feature Status tab in the top menu
to be useful — since it includes links to the Standards Positions and
CSS Features pages, which docs.webkit.org doesn’t link to (and
probably doesn’t need to).

docs/index.md

@@ -5,7 +5,7 @@ hide:
 
 # WebKit Overview
 
-WebKit is a cross-platform web browser engine. On iOS and macOS, it powers Safari, Mail, iBooks, and many other applications.
+WebKit is a cross-platform web browser engine. On iOS and macOS, it powers Safari, Mail, iBooks, and many other applications. For more information about WebKit, see the [WebKit project website](https://webkit.org/).
 
 ## Getting Up and Running