[DOC-3] Wire Doxygen-documented APIs into RST pages (missing Breathe directives)

[lsawade]

Problem: Several C++ namespaces have Doxygen comments but no Breathe directive in any RST page, so they never appear in the rendered documentation:
specfem::chunk_face — 5 headers, fully documented, zero RST coverage.
specfem::constants — constants (COURANT_NUMBER_SUGGESTED, NPTS_PER_WAVELENGTH, N_SLS, NF_ATTENUATION) have no RST page.
specfem::io_backends — has 5 backend entries but the top-level .. doxygennamespace:: directive is missing.
specfem::tag_dispatch — has 29 function/struct entries but missing the namespace-level directive.
Address plan:
Create docs/sections/api/specfem/chunk_face/index.rst with .. doxygennamespace:: specfem::chunk_face.
Create docs/sections/api/specfem/constants/index.rst with .. doxygennamespace:: specfem::constants (or merge into the utilities page).
Add .. doxygennamespace:: specfem::io_backends to docs/sections/api/specfem/io/backends/index.rst.
Add .. doxygennamespace:: specfem::tag_dispatch to docs/sections/api/specfem/tag_dispatch/index.rst.
Link new pages in the appropriate toctree entries.
Files: docs/sections/api/specfem/chunk_face/index.rst (new), docs/sections/api/specfem/constants/index.rst (new), docs/sections/api/specfem/io/backends/index.rst, docs/sections/api/specfem/tag_dispatch/index.rst