Help: Add release notes for Find{BLAS,LAPACK} BLA_SIZEOF_INTEGER option

Help/release/dev/find-blas-lapack-sizeof-integer.rst

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+find-blas-lapack-sizeof-integer
+-------------------------------
+
+* The :module:`FindBLAS` and :module:`FindLAPACK` modules gained
+  a ``BLA_SIZEOF_INTEGER`` option to find a BLAS/LAPACK whose ABI
+  uses a specific integer size.