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/specs.rst b/docs/specs.rst
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+Specifications
+==============
+
+.. toctree::
+   :maxdepth: 2
+
+   specs/signed-interest
+   specs/certificate-format
+   specs/safe-bag