docs: rebrand Certifai -> Prüfwerk (user-facing name)

The product is renamed Certifai -> Prüfwerk. This changes only the user-facing
brand text in the documentation (docs/**, VitePress title/name). It does NOT
touch functional identifiers that happen to contain "certifai" (the Docker
network, Keycloak realm/client, Harbor image project, the compliance-* crate
names, or the repo name) — those are infra-coupled and need a separate,
coordinated migration.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EgxGHn22YEfQz5fLHSHkLv

docs/index.md

@@ -2,7 +2,7 @@
 layout: home
 
 hero:
-  name: Certifai
+  name: Prüfwerk
   text: AI-Powered Security Compliance
   tagline: Automated SAST, SBOM, DAST, CVE monitoring, and code intelligence for your repositories
   actions: