add section headings

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

docs/3.0-migration-guide.md

@@ -15,6 +15,20 @@ The changes are detailed below, and include:
 - Changing function signatures (e.g., adding return codes or extra parameters); introducing const to arguments.
 - Removal of functions marked as deprecated in 2.x
 
+## Low-level crypto
+
+## High-level crypto
+
+## PSA
+
+## The ALT interface
+
+## X.509
+
+## SSL
+
+## Tooling
+
 ### Introduce a level of indirection and versioning in the config files
 
 `config.h` was split into `build_info.h` and `mbedtls_config.h`.