Restructure docs: docs-first, SSOT, one ADR per decision

- Add docs/README.md as the index of all documentation.
- Split the 52 DEC entries of decisioni-e-limiti.md into docs/adr/ADR-NN
  files (same numbers), add ADR-53 (audit log), TEMPLATE.md and an ADR index
  with a feature -> ADR table.
- Rename decisioni-e-limiti.md to limiti-noti.md (LIM only) and rewrite all
  DEC-nn references as links to the ADRs.
- Document the docs-first / SSOT / ADR rules in processo-di-modifica.md and
  map the docs in CLAUDE.md and README.md.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/api.md

@@ -862,7 +862,7 @@ dei parametri operativi in
 
 - **[API-34]** Esporta la chiave privata WIF dell'indirizzo di un utente, per
   interventi manuali. È una scelta voluta (vedi
-  [decisioni-e-limiti.md](decisioni-e-limiti.md)).
+  [limiti-noti.md](limiti-noti.md)).
   - **Parametro:** `user_id` intero.
   - **Risposta `200`:** `{"address", "wif"}`.
   - **Errori:** `404 user_not_found` (oggi non strutturato, `"user not