wikiheaders: Link to the SDL3 wiki in the preamble.

Reference Issue #6568.

.wikiheaders-options

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