CUDA: expose in docs, drop dead headers

Doxygen never read *.cuh and needs EXTENSION_MAPPING to parse it; add
both, plus C++/Python pages and AARE_CUDA build docs.

Delete ClusterFinderCUDA_old.hpp (declared the same class, polluting
the API page), utils/batch.hpp, and the deck. All unreferenced.

TODO:
- Frozen + CUDAGraph: bound in Python, headers not installed. Ship or drop?
- test_ClusterFinderCUDA.py guarded on _cuda_available()
- AARE_CUDA_ARCHITECTURES=native needs CMake 3.24, project min is 3.15
- make docs somewhere with sphinx+breathe; new .rst files never rendered
- fmt nvcc warnings: gone at -std=c++20 or with -isystem

docs/src/python/cluster/index.rst

@@ -7,5 +7,6 @@ Cluster & Interpolation
 
    pyCluster
    pyClusterVector
+   pyClusterFinderCUDA
    pyInterpolation
    pyVarClusterFinder

docs/src/python/cluster/pyClusterFinderCUDA.rst

@@ -0,0 +1,11 @@
+ClusterFinderCUDA
+=================
+
+.. py:currentmodule:: aare
+
+Requires a build configured with ``-DAARE_CUDA=ON``. On a CPU-only build the
+factory raises ``RuntimeError``.
+
+.. autofunction:: ClusterFinderCUDA
+
+.. autofunction:: find_cluster_views_batched_iter