Extract Assignments and Approval guide text to an Approval informational section (#1638)

* move validity module to disputes module

* prepare a section on approvals

* extract approval/assignments text to an overview section

* Apply suggestions from code review

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Co-authored-by: Fedor Sakharov <fedor.sakharov@gmail.com>

polkadot/roadmap/implementers-guide/src/SUMMARY.md

@@ -5,15 +5,16 @@
 - [Whence Parachains](whence-parachains.md)
 - [Parachains Overview](parachains-overview.md)
 - [Architecture Overview](architecture.md)
+- [Approval Overview](approval.md)
 - [Messaging Overview](messaging.md)
 - [Runtime Architecture](runtime/README.md)
   - [Initializer Module](runtime/initializer.md)
   - [Configuration Module](runtime/configuration.md)
+  - [Disputes Module](runtime/disputes.md)
   - [Paras Module](runtime/paras.md)
   - [Scheduler Module](runtime/scheduler.md)
   - [Inclusion Module](runtime/inclusion.md)
   - [InclusionInherent Module](runtime/inclusioninherent.md)
-  - [Validity Module](runtime/validity.md)
   - [Router Module](runtime/router.md)
 - [Runtime APIs](runtime-api/README.md)
   - [Validators](runtime-api/validators.md)
@@ -40,9 +41,6 @@
     - [Availability Distribution](node/availability/availability-distribution.md)
     - [Bitfield Distribution](node/availability/bitfield-distribution.md)
     - [Bitfield Signing](node/availability/bitfield-signing.md)
-  - [Validity](node/validity/README.md)
-    - [Approvals](node/validity/approvals.md)
-    - [Approval assignments](node/validity/assignmets.md)
   - [Utility Subsystems](node/utility/README.md)
     - [Availability Store](node/utility/availability-store.md)
     - [Candidate Validation](node/utility/candidate-validation.md)