[FAB-8562] Add Identity.md to TOC

Adding Identity.md to TOC. Also moving both identity and
membership docs in TOC so that they're back to back.
[ci-skip]

Change-Id: I59e1edc15201109bdd3179459a73dd63ac274c85
Signed-off-by: joe-alewine <Joe.Alewine@ibm.com>

Author: joe-alewine

docs/source/index.rst

@@ -44,8 +44,9 @@ Business:
    blockchain
    functionalities
    fabric_model
-   ledger
+   identity/identity.md
    membership/membership.md
+   ledger
    usecases
 
 .. toctree::