doc: move file system into os services

Move this documentation to a new location under OS Services section.

See #33725

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/doc/reference/index.rst b/doc/reference/index.rst
index f59685b..94b965e 100644
--- a/doc/reference/index.rst
+++ b/doc/reference/index.rst
@@ -14,7 +14,6 @@
    devicetree/index.rst
    display/index.rst
    edac/index.rst
-   file_system/index.rst
    iterable_sections/index.rst
    libc/index.rst
    misc/index
diff --git a/doc/reference/file_system/index.rst b/doc/services/file_system/index.rst
similarity index 100%
rename from doc/reference/file_system/index.rst
rename to doc/services/file_system/index.rst
diff --git a/doc/services/index.rst b/doc/services/index.rst
index 41a9553..af6e880 100644
--- a/doc/services/index.rst
+++ b/doc/services/index.rst
@@ -8,6 +8,7 @@
 
 
    debugging/index.rst
+   file_system/index.rst
    formatted_output.rst
    logging/index.rst
    tracing/index.rst