[K/N] Add design overview to custom_alloc/README and rename page types ^KT-55364

Co-authored-by: Troels Lund <troels@google.com>

Merge-request: KOTLIN-MR-626
Merged-by: Alexander Shabalin <alexander.shabalin@jetbrains.com>

kotlin-native/runtime/src/custom_alloc/cpp/ExtraObjectPageTest.cpp

@@ -11,7 +11,6 @@
 #include "ExtraObjectData.hpp"
 #include "ExtraObjectPage.hpp"
 #include "gtest/gtest.h"
-#include "TypeInfo.h"
 
 namespace {