Enable the targets chapter

Author: AfoHT

book/en/src/SUMMARY.md

@@ -29,6 +29,7 @@
   - [v0.4.x to v0.5.x](./migration/migration_v4.md)
   - [RTFM to RTIC](./migration/migration_rtic.md)
 - [Under the hood](./internals.md)
+  - [Cortex-M architectures](./internals/targets.md)
   <!--- [Interrupt configuration](./internals/interrupt-configuration.md)-->
   <!--- [Non-reentrancy](./internals/non-reentrancy.md)-->
   <!--- [Access control](./internals/access.md)-->