Publish "Detailed error explanations" design documentation page (#3780)

The page was added back in #3734, but was not added to the index, so

https://smithy-lang.github.io/smithy-rs/design/client/detailed_error_explanations.html,
which is exposed here:


https://github.com/smithy-lang/smithy-rs/blob/10a92052b1ca4f6442b15f544dd2940f613bd008/rust-runtime/aws-smithy-runtime-api/src/client/connection.rs#L24-L27

currently points to a 404.

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design/src/SUMMARY.md

@@ -13,6 +13,7 @@
 - [Client](./client/overview.md)
   - [What is the 'orchestrator' and why does it exist?](./client/orchestrator.md)
   - [Identity and Auth](./client/identity_and_auth.md)
+  - [Detailed error explanations](./client/detailed_error_explanations.md)
 
 - [Server](./server/overview.md)
   - [Middleware](./server/middleware.md)