docs: clarify and cleanup the certificate format spec

Also, cleanup references to the legacy signed interest spec

Change-Id: Ib761399b6e44cefea6331e4e128bb2976c2fc11d
diff --git a/docs/specs.rst b/docs/specs.rst
index 65803db..2d4a5ab 100644
--- a/docs/specs.rst
+++ b/docs/specs.rst
@@ -4,7 +4,7 @@
 .. toctree::
    :maxdepth: 2
 
-   specs/signed-interest
-   specs/certificate-format
+   specs/certificate
    specs/safe-bag
    specs/validation-error-code
+   specs/signed-interest