book: create learn section for zk-explainer and move tutorial up

doc/src/SUMMARY.md

@@ -3,8 +3,7 @@
 [DarkFi](README.md)
 - [Ideology](IDEOLOGY.md)
 - [Development](development.md)
-- [Tutorial](tutorial/tutorial.md)
-  - [ZK explainer](tutorial/zk_explainer.md)
+- [Tutorial](tutorial.md)
 - [Architecture](architecture/architecture.md)
   - [Overview](architecture/overview.md)
   - [Blockchain](architecture/blockchain.md)
@@ -23,3 +22,5 @@
   - [ircd](misc/ircd.md)
   - [tau](misc/tau.md)
   - [dnetview](misc/dnetview.md)
+- [Learn](learn/learn.md)
+  - [ZK explainer](tutorial/zk_explainer.md)

doc/src/learn/learn.md

@@ -0,0 +1,3 @@
+# Learn
+
+This section contains learning resources related to DarkFi.