docs: document DEDX_DEFAULT and DEDX_AUTO programs, and document available materials #145

Context

README.md's "Supported programs" table and its mirror in index.rst (pulled into docs/source/index.rst via .. include::) both list DEDX_PSTAR, DEDX_ASTAR, DEDX_MSTAR, DEDX_ICRU49, DEDX_ICRU73, DEDX_ICRU73_OLD, DEDX_BETHE_EXT00, and DEDX_ICRU — but DEDX_DEFAULT and DEDX_AUTO are missing from both. DEDX_AUTO is new (#144); DEDX_DEFAULT has existed for a while but was apparently never documented at this level (only in the dedx.h Doxygen comments).

Separately, there is no documented list of available materials anywhere — not in README.md, not in index.rst, not in the Doxygen-generated API reference. The only place material identifiers exist is the include/dedx_elements.h enum; a reader has to go spelunking in a header to find out what DEDX_WATER, DEDX_BORON_CARBIDE, etc. even are, or which ones are usable with which program.

What to add

1. DEDX_DEFAULT and DEDX_AUTO rows in the programs table (both README.md and index.rst):

• DEDX_DEFAULT — currently behaves identically to DEDX_BETHE_EXT00 in find_data() (src/dedx.c); worth explicitly saying whether it's meant as a stable alias/default entry point or is expected to diverge later, since right now a reader has no way to tell why both exist.
• DEDX_AUTO — auto-selects the best tabulated ICRU report for the ion, like DEDX_ICRU, but falls back to the Bethe-Bloch formula for elements/compounds nothing tabulated covers (comparable to the AUTO mode in dedx_web). Should call out explicitly that this is different from DEDX_ICRU, which never falls back and still fails with DEDX_ERR_COMBINATION_NOT_FOUND for combinations no ICRU sub-report covers — this distinction was a deliberate design decision (see the discussion on Add proper handling of missing ion+material+program combinations #51) and is exactly the kind of thing a reader would otherwise miss.
• Both should note the caveat already present for DEDX_BETHE_EXT00-style programs (advisory-only energy bounds from dedx_get_min_energy()/dedx_get_max_energy(); the real bounds come from whichever tier actually resolves the request).

2. Document available materials well, likely as a new section/page:

• A materials table or generated list (elements + compounds), since right now the only source of truth is the include/dedx_elements.h enum.
• Documentation of the two ways to query availability, and when to use which:
  • dedx_get_material_list(program) — coarse, per-program hint (not ion-specific, doesn't account for per-compound decomposition feasibility).
  • dedx_get_material_list_for_ion(program, ion, ...) / dedx_fill_material_list_for_ion(...) (added in Fix stale material-availability tables; add ion-aware lookup and Bethe fallback for DEDX_ICRU #144) — accurate, ion-specific, decomposition-aware answer that mirrors what dedx_load_config() will actually do.
• Tie this together with the existing README/index.rst note about Bragg/stoichiometric fallback for compounds not natively covered by a program's own database.

Suggested locations

• README.md: extend the existing "Supported programs" table + note.
• index.rst (top-level, included into docs/source/index.rst): same table, plus a new subsection (e.g. under "Supported programs" or as its own "Available materials" section) for the materials list/API guidance.
• Related but broader: docs: add high-level "purpose, role & how to read the numbers" guide #116 (docs epic for a full "purpose, role & how to read the numbers" guide) already proposes a program-coverage table; this issue is the narrower, concrete piece of that specifically covering the DEDX_DEFAULT/DEDX_AUTO gap and materials documentation, prompted directly by Fix stale material-availability tables; add ion-aware lookup and Bethe fallback for DEDX_ICRU #144 landing.

References

• Add proper handling of missing ion+material+program combinations #51 — original issue (missing ion+material+program combination handling)
• Fix stale material-availability tables; add ion-aware lookup and Bethe fallback for DEDX_ICRU #144 — added DEDX_AUTO, dedx_get_material_list_for_ion()/dedx_fill_material_list_for_ion(), and fixed the stale per-program materials table
• docs: add high-level "purpose, role & how to read the numbers" guide #116 — broader docs epic proposing a program-coverage table (predates DEDX_AUTO)