Remove reference to gul14.info

[why]
The gul14.info will expire in a year or so.

[how]
Replace gul14.info by gul-cpp.github.io/gul14.

Signed-off-by: Lars Froehlich <lars.froehlich@desy.de>

Author: alt-graph

README.md

@@ -14,7 +14,8 @@ by the library are:
   * Core guideline support library (GSL) excerpts
   * Backports from new standard library extensions
 
-To get started, have a look at the [documentation website](https://gul14.info/)
+To get started, have a look at the
+[documentation website](https://gul-cpp.github.io/gul14/).
 
 Here you find more detailed information on: