book: add reference links section (#1345)

book/src/SUMMARY.md

@@ -18,4 +18,8 @@
 - [Anchor Periphery](./chapter_4/anchor_periphery.md)
   - [CLI](./chapter_4/cli.md)
   - [IDL]()
-- [Anchor BTS]()
+- [Anchor BTS]()
+
+---
+
+- [Reference Links](./reference_links.md)

book/src/reference_links.md

@@ -0,0 +1,5 @@
+# Reference Links
+
+- [Accounts Reference](https://docs.rs/anchor-lang/latest/anchor_lang/accounts/index.html)
+- [Constraints Reference](https://docs.rs/anchor-lang/latest/anchor_lang/derive.Accounts.html)
+- [Error Codes](https://docs.rs/anchor-lang/latest/anchor_lang/__private/enum.ErrorCode.html)