docs: move gcc-plugins to the kbuild manual

Information about GCC plugins is relevant to kernel building, so move this
document to the kbuild manual.
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/index.rst

@@ -102,7 +102,6 @@ Documents that don't fit elsewhere or which have yet to be categorized.
    :maxdepth: 1
 
    librs
-   gcc-plugins
    ioctl
 
 .. only:: subproject and html

Documentation/kbuild/index.rst

@@ -19,6 +19,7 @@ Kernel Build System
 
     issues
     reproducible-builds
+    gcc-plugins
 
 .. only::  subproject and html
 

MAINTAINERS

@@ -6934,7 +6934,7 @@ S:	Maintained
 F:	scripts/gcc-plugins/
 F:	scripts/gcc-plugin.sh
 F:	scripts/Makefile.gcc-plugins
-F:	Documentation/core-api/gcc-plugins.rst
+F:	Documentation/kbuild/gcc-plugins.rst
 
 GASKET DRIVER FRAMEWORK
 M:	Rob Springer <rspringer@google.com>

scripts/gcc-plugins/Kconfig

@@ -23,7 +23,7 @@ menuconfig GCC_PLUGINS
 	  GCC plugins are loadable modules that provide extra features to the
 	  compiler. They are useful for runtime instrumentation and static analysis.
 
-	  See Documentation/core-api/gcc-plugins.rst for details.
+	  See Documentation/kbuild/gcc-plugins.rst for details.
 
 if GCC_PLUGINS