[FAB-8171] Ordering service concept doc

Conceptual information on orderers and the
ordering service generally, and then a
discussion of implementations, focusing on
Raft.

Change-Id: I679594b542d1496f9ee7b566f70f73ba84734607
Signed-off-by: joe-alewine <Joe.Alewine@ibm.com>
Signed-off-by: David Enyeart <enyeart@us.ibm.com>

Author: joe-alewine

docs/source/key_concepts.rst

@@ -11,7 +11,8 @@ Key Concepts
    identity/identity.md
    membership/membership.md
    peers/peers.md
-   private-data/private-data.md
-   ledger/ledger.md
    smartcontract/smartcontract.md
+   ledger/ledger.md
+   orderer/ordering_service.md
+   private-data/private-data.md
    usecases