[GLib] Install documentation into ${CMAKE_INSTALL_DATADIR}/doc

https://bugs.webkit.org/show_bug.cgi?id=265133

Reviewed by Carlos Garcia Campos.

Projects that use gi-docgen do not generally install into the old
gtk-doc directory. Probably time to update our installation location.

* Source/cmake/FindGIDocgen.cmake:

Canonical link: https://commits.webkit.org/271014@main

Source/cmake/FindGIDocgen.cmake

@@ -231,7 +231,7 @@ function(GI_DOCGEN namespace toml)
 
     install(
         DIRECTORY "${outdir}"
-        DESTINATION "${CMAKE_INSTALL_DATADIR}/gtk-doc/html"
+        DESTINATION "${CMAKE_INSTALL_DATADIR}/doc"
         COMPONENT development
     )