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>
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index 322ac954b..da0ed97 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -36,6 +36,7 @@
    memory-hotplug
    protection-keys
    ../RCU/index
+   gcc-plugins
 
 
 Interfaces for kernel debugging