Implementer's guide: Approval Voting Subsystem (#1691)

* add storage for approvals module

* basics of approval logic

* fix links

* Session info module

* create an approvals_inherent module

* integrate approvals module with inclusion

* Remove approvals runtime

* tweak mentions of on-chain logic

* add note on finality-grandpa voting rule

* elaborate on node-side components

* stub for availability recovery

* add another note on voting rule

* Beginnings of approval subsystems

* flesh out approval voting now

* logic for checking assignment certs

* initial scheduler logic

* scheduler logic

* adjst tranche taking logic

* approval voting import

* approval work (voting side)

* amend some TODOs

* mark some TODOs

* describe `ApprovedAncestor`

* reference protocol-approval.md

* clarity on bitfield

* remove approvals_inherent

* tweak session_info module according to review

* formatting & nits

Co-authored-by: Robert Habermeier <robert@Roberts-MacBook-Pro.local>

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

@@ -8,6 +8,7 @@
 - [Architecture Overview](architecture.md)
   - [Messaging Overview](messaging.md)
 - [Runtime Architecture](runtime/README.md)
+  - [ApprovalsInherent Module](runtime/approvals_inherent.md)
   - [Initializer Module](runtime/initializer.md)
   - [Configuration Module](runtime/configuration.md)
   - [Disputes Module](runtime/disputes.md)
@@ -16,6 +17,7 @@
   - [Inclusion Module](runtime/inclusion.md)
   - [InclusionInherent Module](runtime/inclusioninherent.md)
   - [Router Module](runtime/router.md)
+  - [Session Info Module](runtime/session_info.md)
 - [Runtime APIs](runtime-api/README.md)
   - [Validators](runtime-api/validators.md)
   - [Validator Groups](runtime-api/validator-groups.md)
@@ -39,8 +41,13 @@
     - [PoV Distribution](node/backing/pov-distribution.md)
   - [Availability Subsystems](node/availability/README.md)
     - [Availability Distribution](node/availability/availability-distribution.md)
+    - [Availability Recovery](node/availability/availability-recovery.md)
     - [Bitfield Distribution](node/availability/bitfield-distribution.md)
     - [Bitfield Signing](node/availability/bitfield-signing.md)
+  - [Approval Subsystems](node/approval/README.md)
+    - [Approval Voting](node/approval/approval-voting.md)
+    - [Approval Networking](node/approval/approval-networking.md)
+    - [Dispute Participation](node/approval/dispute-participation.md)
   - [Utility Subsystems](node/utility/README.md)
     - [Availability Store](node/utility/availability-store.md)
     - [Candidate Validation](node/utility/candidate-validation.md)
@@ -59,6 +66,7 @@
     - [Chain](types/chain.md)
     - [Messages](types/messages.md)
     - [Network](types/network.md)
+    - [Approvals](types/approval.md)
 
 [Glossary](glossary.md)
 [Further Reading](further-reading.md)