docs: refocus README on users, add quick_start + tutorial, fill CONTRIBUTING

- README.md: pruned developer-oriented sections (Sphinx setup, Qt
  Creator workflow, VSCode debugging, release procedure, AppImage
  Wayland note) and replaced them with a user-facing layout: pre-built
  releases pointer, quick start, manual install, troubleshooting,
  licence.
- CONTRIBUTING.md: absorbed the developer content (debugging in VSCode,
  Qt GUI regen, Sphinx build, validation suite — batch + GUI variants,
  cross-distrib check, release procedure).
- doc/quick_start.md: 5-minute path from install to a passing test,
  in batch mode and in the GUI.
- doc/tutorial.md: guided walk-through against a small calc.py
  module — check, py_func, expected_result, $(...) expansion, group,
  let, condition, report (with the mkdir reminder), context_id.
- CLAUDE.md: subprocess API contract, bins.py, report-exporter
  plugin section, packaging matrix (wheel / PyInstaller / Flatpak /
  .deb work-in-progress), refreshed recent-fixes list. README/CLAUDE
  validation command no longer carries the spurious "-l" flag (which
  is GUI-only and a no-op in batch).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CLAUDE.md

@@ -242,7 +242,7 @@ The `.deb` work-in-progress lives in `package/deb/`:
 ## Validation tests
 Located in `test/validation/`. Run with `-b` flag:
 ```
-./run.sh -b -l mon_log.log -- test/validation/main.tum
+./run.sh -b -- test/validation/main.tum
 ```
 Parallel item tests: `test/validation/items/parallel/test.tum`