docs: add high-level "purpose, role & how to read the numbers" guide

[grzanka]

Part of #108 (health-review epic). Subsumes the units concern in #48.

Context

The reference docs and README are accurate but assume domain knowledge. A new user (or a web/Python consumer) cannot easily answer: what is this, what does each program cover, which units do I get, and when is a number native vs. extended?

Proposed change

Add a top-level conceptual section (in index.rst / docs landing) covering:

• What dE/dx / stopping power is, in plain terms, and what libdedx is for.
• The ecosystem: C core → WASM web app → planned Python wrapper, and how they relate.
• Program coverage table: for each program (PSTAR/ASTAR/MSTAR/ICRU49/ICRU73/Bethe) — supported ions, valid energy range, and source.
• Units, stated unambiguously: mass stopping power MeV cm²/g (note the easy MeV g/cm² inversion mistake, Check mass stopping power units #48), energy MeV/nucl, CSDA range g/cm² — with a worked example.
• Native vs. Bragg-extended results caveat (link Make Bragg/stoichiometric fallback transparent (surface bragg_used) and add accuracy tests #111).

Acceptance criteria

• A conceptual overview page exists and is linked from the docs landing page and README.
• Units for every public quantity are documented with a worked example.
• Per-program coverage (ions + energy range + source) is tabulated.
• The native-vs-fallback caveat is explained and cross-linked.

References

• Check mass stopping power units #48 (units confusion), Make Bragg/stoichiometric fallback transparent (surface bragg_used) and add accuracy tests #111 (fallback transparency), Add man pages #28 (man pages — separate)