.wikiheaders-options: Move SDL2 documentation to a subdirectory.

SDL3 will be the main thing now.

.wikiheaders-options

@@ -1,7 +1,7 @@
-projectfullname = SDL_mixer
-projectshortname = SDL_mixer
+projectfullname = SDL2
+projectshortname = SDL2
 incsubdir = include
-wikisubdir =
+wikisubdir = SDL2
 apiprefixregex = (SDL_|SDLK_|KMOD_|AUDIO_)
 mainincludefname = SDL.h
 versionfname = include/SDL_version.h
@@ -10,6 +10,7 @@ versionminorregex = \A\#define\s+SDL_MINOR_VERSION\s+(\d+)\Z
 versionpatchregex = \A\#define\s+SDL_PATCHLEVEL\s+(\d+)\Z
 selectheaderregex = \ASDL.*?\.h\Z
 projecturl = https://libsdl.org/
-wikiurl = https://wiki.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.)