Add a migration guide for the ROCm-packaged Fortran bindings

[amontoison]

Adds docs/how-to/migration-guide.rst, a how-to for users moving from hipFORT to the Fortran bindings that ship inside ROCm (rocm-systems and rocm-libraries), in the spirit of the layout previewed in #540: one self-contained module per library, ROCm only.

What the page covers

• The two edits. Rename the use statements (use hipfort_rocblas becomes use rocblas, helper modules folded in), and link the per-library Fortran archive (find_package(rocblas) gives roc::rocblas_fortran). Both as tables, plus a sed recipe for the rename and an explicit list of the cases it deliberately leaves to review (two use hip lines, files that used only hipfort_check, roctx and cuda_errors, fixed form, preprocessor guards).
• Build options. BUILD_FORTRAN_BINDINGS, BUILD_FORTRAN_TESTS, and the tri-state FORTRAN_ARRAY_INTERFACES that replaces HIPFORT_USE_FPOINTER_INTERFACES plus HIPFORT_ASSUMED_RANK, with a table mapping every HIPFORT_* option to its successor.
• Compiler support. Why a .mod is compiler- and version-specific, the per-compiler install layout, raw link lines for amdflang and gfortran, and how to rebuild one library's binding from source without rebuilding the C library.
• What does not change, the special cases (in-tree vendor modules, CUDA), and an FAQ including the old/new coexistence matrix and the fixed-form question.

Also wires the page into docs/index.rst and docs/sphinx/_toc.yml.in under How to.

Status

Draft. The page states a ROCm 10.2 / 11.0 timeline and several conventions that are settled in the design document but not yet implemented in rocm-systems / rocm-libraries, so the content should be reviewed against whatever actually lands there.

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