implementers-guide: in TOC move collators before backing, to match protocol pipeline (#1611)

roadmap/implementers-guide/src/SUMMARY.md

@@ -28,6 +28,9 @@
 - [Node Architecture](node/README.md)
   - [Subsystems and Jobs](node/subsystems-and-jobs.md)
   - [Overseer](node/overseer.md)
+  - [Collators](node/collators/README.md)
+    - [Collation Generation](node/collators/collation-generation.md)
+    - [Collator Protocol](node/collators/collator-protocol.md)
   - [Backing Subsystems](node/backing/README.md)
     - [Candidate Selection](node/backing/candidate-selection.md)
     - [Candidate Backing](node/backing/candidate-backing.md)
@@ -37,9 +40,6 @@
     - [Availability Distribution](node/availability/availability-distribution.md)
     - [Bitfield Distribution](node/availability/bitfield-distribution.md)
     - [Bitfield Signing](node/availability/bitfield-signing.md)
-  - [Collators](node/collators/README.md)
-    - [Collation Generation](node/collators/collation-generation.md)
-    - [Collator Protocol](node/collators/collator-protocol.md)
   - [Validity](node/validity/README.md)
     - [Approvals](node/validity/approvals.md)
     - [Approval assignments](node/validity/assignmets.md)