docs: set up MkDocs site with technical/general docs split

Add mkdocs.yml (windmill theme, mermaid2, matching ~/Chirone/chirone/etl
setup) with nav split into "ThothII (Documentazione Tecnica)" — architecture
overview, existing design specs/plans, reports — and "Considerazioni
Generali" for cross-project notes.

Add docs/general/pi-configuration.md explaining Pi's three model-resolution
tiers (built-in, user models.json, project extension) and where GLM/DeepSeek/
Qwen each sit. Add docs/architecture/overview.md synthesizing the ThothII
architecture for the doc site. Relocate the L2 run report into docs/reports/.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/requirements.txt

@@ -0,0 +1,3 @@
+mkdocs>=1.6
+mkdocs-windmill>=1.0.5
+mkdocs-mermaid2-plugin>=1.2.1