Add Maven site documentation generated from CLAUDE.md

Add Maven site documentation generated from CLAUDE.md

Replace outdated XHTML stubs (copied from a different project) with
proper Markdown documentation covering the StILi architecture and both
import scripts. CLAUDE.md is the SSOT; the site docs are regenerated
from it on demand, not automatically before each release.

Changes:
- src/site/site.xml: rewrite for StILi (maven-fluido-skin, clean nav)
- src/site/markdown/index.md: overview, name origin, quick start,
  community extensibility model
- src/site/markdown/architecture.md: site.vm mechanics, stili-json
  format, page types, body extraction
- src/site/markdown/import-in-hugo.md: full Hugo script reference
- src/site/markdown/import-in-astro.md: full Astro script reference
- src/site/xhtml/: remove four legacy XHTML stubs from Thymeleaf skin
- pom.xml: add doxia-module-markdown dependency, update
  maven-project-info-reports-plugin 2.9 → 3.7.0 with explicit report
  set (index, summary, licenses, team, scm, dependency-info, plugins)
- CLAUDE.md: add documentation section with SSOT contract, file
  mapping, regeneration rules and minimal-diff principle

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>