docs: spec of SafeBag format

This spec defines the TLV-based format of SafeBag, replacing the
previous SecureBag which uses DER-based encoding.

Change-Id: I001153bd3f4246a0f84656167b00c6387b2fea07
Refs: #3048
diff --git a/docs/index.rst b/docs/index.rst
index fcfc521..291fb72 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -18,6 +18,7 @@
    INSTALL
    examples
    tutorials
+   specs
    manpages
 
 .. toctree::
@@ -39,8 +40,12 @@
    + :doc:`tutorials/security-library`
    + :doc:`tutorials/utils-ndn-regex`
    + :doc:`tutorials/security-validator-config`
-   + :doc:`tutorials/signed-interest`
-   + :doc:`tutorials/certificate-format`
+
+- :doc:`specs`
+
+   + :doc:`specs/signed-interest`
+   + :doc:`specs/certificate-format`
+   + :doc:`specs/safe-bag`
 
 - :doc:`manpages`