Moving all section headers up on the hierarchy

Author: JOOpdenhoevel

release_notes.md

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+# What's new in StencilStream 4.0.0
+
+StencilStream 4.0.0 broadens the framework from an FPGA-focused stencil
+accelerator into a portable, SYCL-based 2D stencil framework that targets FPGAs,
+NVIDIA GPUs, and CPUs from the same transition-function code. It also brings a
+significant API cleanup, makes the FPGA backends faster through spatial
+parallelism, and adds an experimental multi-FPGA execution path.
+
+## New backends and performance features
+
+* **GPU backend.** A new CUDA backend, built on
+  [Codeplay's oneAPI for NVIDIA GPUs plugin](https://developer.codeplay.com/products/oneapi/nvidia/home/index.html),
+  brings StencilStream to NVIDIA GPUs. A transparent
+  Array-of-Structs ↔ Struct-of-Arrays transformation lets the very same
+  transition function reach high throughput on both GPUs and FPGAs. See the
+  IWOCL '26 publication ([doi:10.1145/3811257.3811259](https://doi.org/10.1145/3811257.3811259))
+  for a detailed evaluation.
+* **Spatial parallelism on the FPGA backends.** The Monotile and Tiling
+  backends now allow vectorizing the I/O and update kernels, raising the achievable
+  throughput substantially over 3.0.0. The highest single-device throughput
+  measured for the new release is 176.08 GCells/s (1.58 TFLOPS) for Jacobi on
+  the Tiling backend, and 122.67 GCells/s (1.84 TFLOPS) arithmetic throughput for HotSpot on the
+  Monotile backend (BittWare 520N / Intel Stratix 10 GX 2800).
+* **Experimental multi-FPGA Monotile backend.** Uses the networking
+  capabilities of high-end FPGAs to scale a Monotile design beyond a single
+  device.
+
+## Breaking API changes
+
+These changes affect every user upgrading from 3.0.0:
+
+* **Index types.** The configurable `stencil::uindex_t` and `stencil::index_t`,
+  along with the `STENCIL_INDEX_WIDTH` macro, have been replaced by
+  `std::size_t` and `std::ptrdiff_t` to align with the SYCL standard. Index width
+  narrowing is now done automatically within the FPGA backends.
+  The `StencilStream/Index.hpp` header has been removed.
+* **2D coordinates.** The custom `ID` / `UID` / `GenericID` types have been
+  replaced by `sycl::id<2>` and `sycl::range<2>`, again to align with the SYCL standard.
+   The `StencilStream/GenericID.hpp` header has been removed.
+* **Index ordering.** The first index of a 2D coordinate is now the row and the
+  second is the column, again matching the SYCL standard. Transition functions,
+  grid construction, and accessor calls written against 3.0.0 must be updated
+  accordingly.
+* **No more Boost dependency.** StencilStream no longer pulls in Boost;
+  builds and downstream projects can drop the corresponding find/link lines.
+* **Internal headers reorganized.** Implementation-detail headers (helpers,
+  I/O / memory / switch kernels, the per-backend kernel and design classes)
+  now live under `StencilStream/internal/` and per-backend `internal/`
+  subdirectories. Public concepts in `Concepts.hpp` and `Stencil.hpp` have
+  been updated to use the new index types.
+
+## New example and documentation
+
+* **Jacobi example.** A new example under `examples/jacobi/` provides multiple
+  Jacobi-kernel variants with adjustable computational complexity, and serves
+  as the primary benchmark in the new performance figures.
+* **Documentation overhaul.** The README has been rewritten with up-to-date
+  build, run, and benchmarking instructions covering all backends, and the
+  Doxygen documentation now uses the Doxygen Awesome theme with a dark-mode
+  toggle.
+
+## Build and tooling
+
+* **Toolchain.** Validated on Intel oneAPI 24.2.1.
+* **Environment setup.** Separate `scripts/env_fpga.sh` and
+  `scripts/env_cuda.sh` scripts replace the previous combined setup, so the
+  FPGA and CUDA toolchains can be loaded independently on Noctua 2.
+* **Per-backend benchmark scripts.** Each example now ships
+  `benchmark_mono.sh`, `benchmark_tiling.sh`, and `benchmark_cuda.sh` driver
+  scripts on top of the shared Julia benchmark harness.
+* **Standalone Conway build.** The Conway example provides a
+  `CMakeLists.standalone.txt` that can be used to build it outside of the
+  StencilStream source tree.