[libc++] [doc] Add issue tracking for spaceship operator<=> implementation

Add issue tracking and assignment for the implementation of P1614R2: The Mothership has Landed.

Reviewed By: cjdb, #libc, Mordante, Quuxplusone

Differential Revision: https://reviews.llvm.org/D107877

NOKEYCHECK=True
GitOrigin-RevId: 81507bcf6b35801beaeefa77cc1b0148e56fac36
diff --git a/docs/index.rst b/docs/index.rst
index b13e42f..a4bbac7 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -43,8 +43,9 @@
    Status/Cxx17
    Status/Cxx20
    Status/Cxx2b
-   Status/Ranges
    Status/Format
+   Status/Ranges
+   Status/Spaceship
 
 
 .. toctree::