docs(sdk): add dedicated Core Types page and update SUMMARY to avoid anchor doctest errors

Author: hoffmabc

src/SUMMARY.md

@@ -48,7 +48,7 @@
     - [Rust API Reference](sdk/rust/api-reference.md)
     - [Rust Examples](sdk/rust/examples.md)
     - [Program Development](sdk/rust/program-development.md)
-  - [Core Types](sdk/sdk.md#shared-concepts)
+  - [Core Types](sdk/core-types.md)
     - [Pubkey](sdk/pubkey.md)
     - [Account](sdk/account.md)
     - [Instructions and Messages](sdk/instructions-and-messages.md)

src/sdk/core-types.md

@@ -0,0 +1,12 @@
+# Core Types
+
+These core types and concepts are shared by both SDKs and used throughout the docs:
+
+- [Pubkey](pubkey.md) - Public key type for identifying accounts
+- [Account](account.md) - Account structure and management
+- [Instructions and Messages](instructions-and-messages.md) - Transaction building
+- [Runtime Transaction](runtime-transaction.md) - Transaction format
+- [Processed Transaction](processed-transaction.md) - Transaction results
+- [Signature](signature.md) - Digital signatures
+
+See also: [SDK Overview](sdk.md).