C++: Add change notes.

Author: geoffw0

cpp/ql/lib/change-notes/2024-06-14-boost-asio.md

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+---
+category: minorAnalysis
+---
+* A partial model for the `Boost.Asio` network library has been added. This includes sources, sinks and summaries for certain functions in `Boost.Asio`, such as `read_until` and `write`.

cpp/ql/lib/change-notes/2024-06-14-models-as-data-yml-extensions.md

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+---
+category: feature
+---
+* Data models can now be added with data extensions. In this way source, sink and summary models can be added in extension `.model.yml` files, rather than by writing classes in QL code. New models should be added in the `lib/ext` folder.