wikiheaders: Link to the SDL2 wiki in the preamble.

Fixes #6568.

.wikiheaders-options

@@ -13,4 +13,4 @@ projecturl = https://libsdl.org/
 wikiurl = https://wiki.libsdl.org
 bugreporturl = https://github.com/libsdl-org/sdlwiki/issues/new
 warn_about_missing = 0
-wikipreamble = (This is the documentation for SDL3, which is under heavy development and the API is changing! SDL2 is the current stable version!)
+wikipreamble = (This is the documentation for SDL3, which is under heavy development and the API is changing! [SDL2](https://wiki.libsdl.org/) is the current stable version!)