Update docs/SUMMARY.md (#8665)

## Problem

This page had many dead links, and was confusing for folks looking for
documentation about our product.

Closes: https://github.com/neondatabase/neon/issues/8535

## Summary of changes

- Add a link to the product docs up top
- Remove dead/placeholder links

docs/SUMMARY.md

@@ -1,13 +1,18 @@
 # Summary
 
+# Looking for `neon.tech` docs?
+
+This page linkes to a selection of technical content about the open source code in this repository.
+
+Please visit https://neon.tech/docs for documentation about using the Neon service, which is based on the code
+in this repository.
+
+# Architecture
+
 [Introduction]()
 - [Separation of Compute and Storage](./separation-compute-storage.md)
 
-# Architecture
-
 - [Compute]()
-  - [WAL proposer]()
-  - [WAL Backpressure]()
   - [Postgres changes](./core_changes.md)
 
 - [Pageserver](./pageserver.md)
@@ -16,33 +21,15 @@
     - [WAL Redo](./pageserver-walredo.md)
     - [Page cache](./pageserver-pagecache.md)
     - [Storage](./pageserver-storage.md)
-        - [Datadir mapping]()
-        - [Layer files]()
-        - [Branching]()
-        - [Garbage collection]()
-    - [Cloud Storage]()
     - [Processing a GetPage request](./pageserver-processing-getpage.md)
     - [Processing WAL](./pageserver-processing-wal.md)
-	- [Management API]()
-	- [Tenant Rebalancing]()
 
 - [WAL Service](walservice.md)
   - [Consensus protocol](safekeeper-protocol.md)
-  - [Management API]()
-  - [Rebalancing]()
-
-- [Control Plane]()
-
-- [Proxy]()
 
 - [Source view](./sourcetree.md)
   - [docker.md](./docker.md) — Docker images and building pipeline.
   - [Error handling and logging](./error-handling.md)
-  - [Testing]()
-    - [Unit testing]()
-    - [Integration testing]()
-    - [Benchmarks]()
-
 
 - [Glossary](./glossary.md)
 
@@ -58,28 +45,6 @@
 
 # RFCs
 
-- [RFCs](./rfcs/README.md)
-
-- [002-storage](rfcs/002-storage.md)
-- [003-laptop-cli](rfcs/003-laptop-cli.md)
-- [004-durability](rfcs/004-durability.md)
-- [005-zenith_local](rfcs/005-zenith_local.md)
-- [006-laptop-cli-v2-CLI](rfcs/006-laptop-cli-v2-CLI.md)
-- [006-laptop-cli-v2-repository-structure](rfcs/006-laptop-cli-v2-repository-structure.md)
-- [007-serverless-on-laptop](rfcs/007-serverless-on-laptop.md)
-- [008-push-pull](rfcs/008-push-pull.md)
-- [009-snapshot-first-storage-cli](rfcs/009-snapshot-first-storage-cli.md)
-- [009-snapshot-first-storage](rfcs/009-snapshot-first-storage.md)
-- [009-snapshot-first-storage-pitr](rfcs/009-snapshot-first-storage-pitr.md)
-- [010-storage_details](rfcs/010-storage_details.md)
-- [011-retention-policy](rfcs/011-retention-policy.md)
-- [012-background-tasks](rfcs/012-background-tasks.md)
-- [013-term-history](rfcs/013-term-history.md)
-- [014-safekeepers-gossip](rfcs/014-safekeepers-gossip.md)
-- [014-storage-lsm](rfcs/014-storage-lsm.md)
-- [015-storage-messaging](rfcs/015-storage-messaging.md)
-- [016-connection-routing](rfcs/016-connection-routing.md)
-- [017-timeline-data-management](rfcs/017-timeline-data-management.md)
-- [018-storage-messaging-2](rfcs/018-storage-messaging-2.md)
-- [019-tenant-timeline-lifecycles](rfcs/019-tenant-timeline-lifecycles.md)
-- [cluster-size-limits](rfcs/cluster-size-limits.md)
+Major changes are documented in RFCS:
+- See [RFCs](./rfcs/README.md) for more information
+- view the RFCs at https://github.com/neondatabase/neon/tree/main/docs/rfcs