C++: Add change notes.

Author: geoffw0

cpp/ql/lib/change-notes/2024-10-04-getc.md

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+---
+category: minorAnalysis
+---
+* Source models have been added for the standard library function `getc` (and variations).

cpp/ql/lib/change-notes/2024-10-04-models-as-data.md

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+---
+category: feature
+---
+* Models-as-Data support has been added for C/C++. This feature allows flow sources, sinks and summaries to be expressed in compact strings as an alternative to modelling each source / sink / summary with explicit CodeQL. See `dataflow/ExternalFlow.qll` for documentation and specification of the model format, and `models/implementations/ZMQ.qll` for a simple example of models. Importing models from `.yml` is not yet supported.

cpp/ql/lib/change-notes/2024-10-04-zmq.md

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+---
+category: minorAnalysis
+---
+* Source, sink and flow models for the ZeroMQ (ZMQ) networking library have been added.