docs: clarify and cleanup the certificate format spec

Also, cleanup references to the legacy signed interest spec

Change-Id: Ib761399b6e44cefea6331e4e128bb2976c2fc11d
diff --git a/docs/index.rst b/docs/index.rst
index ea459fb..5d7364c 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -49,10 +49,10 @@
 
 - :doc:`specs`
 
-   + :doc:`specs/signed-interest`
-   + :doc:`specs/certificate-format`
+   + :doc:`specs/certificate`
    + :doc:`specs/safe-bag`
    + :doc:`specs/validation-error-code`
+   + :doc:`specs/signed-interest`
 
 - :doc:`manpages`