docs: move gcc_plugins.txt to core-api and rename to .rst

The gcc_plugins.txt file is already a ReST file. Move it
to the core-api book while renaming it.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>

Documentation/core-api/index.rst

@@ -36,6 +36,7 @@ Core utilities
    memory-hotplug
    protection-keys
    ../RCU/index
+   gcc-plugins
 
 
 Interfaces for kernel debugging

MAINTAINERS

@@ -6655,7 +6655,7 @@ S:	Maintained
 F:	scripts/gcc-plugins/
 F:	scripts/gcc-plugin.sh
 F:	scripts/Makefile.gcc-plugins
-F:	Documentation/gcc-plugins.txt
+F:	Documentation/core-api/gcc-plugins.rst
 
 GASKET DRIVER FRAMEWORK
 M:	Rob Springer <rspringer@google.com>

scripts/gcc-plugins/Kconfig

@@ -23,7 +23,7 @@ config 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/gcc-plugins.txt for details.
+	  See Documentation/core-api/gcc-plugins.rst for details.
 
 menu "GCC plugins"
 	depends on GCC_PLUGINS