feat(help): restructure /help as user-facing Help Center with Zammad integration

- Replace MkDocs redirect with a branded, SEO-optimised Help Center template
- Add sections: Quick Start, Sources, Destinations, Workflows, FAQ, Support
- Integrate optional Zammad live-chat widget and ticket form
- Add config settings: ZAMMAD_URL, ZAMMAD_CHAT_ENABLED, ZAMMAD_CHAT_ID,
  ZAMMAD_FORM_ENABLED, SUPPORT_EMAIL
- Move MkDocs developer docs from /help to /developer-docs
- Move interactive API docs (Swagger/ReDoc) to /admin/api-docs and /admin/api-redoc
- Add API Docs and Developer Docs links to Admin menu (desktop + mobile)
- Update navigation Help link from /help/ to /help
- Update .env.demo with Zammad configuration examples
- Document new settings in docs/ConfigurationGuide.md
- Rewrite tests to cover new Help Center behaviour

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

.env.demo

@@ -478,3 +478,15 @@ EMBEDDING_MODEL=text-embedding-3-small
 # Maximum tokens to send to the embedding model.  Set below the model's
 # context window (e.g. 8000 for an 8192-token model).
 EMBEDDING_MAX_TOKENS=8000
+
+# **Support / Help Center – Zammad Integration**
+# Base URL of your Zammad instance (required for chat and ticket form).
+# ZAMMAD_URL=https://zammad.example.com
+# Show a live-chat widget on the Help Center page (requires an online Zammad agent).
+# ZAMMAD_CHAT_ENABLED=false
+# Zammad chat topic ID (see Zammad → Channels → Chat → Topics).
+# ZAMMAD_CHAT_ID=1
+# Show a "Submit a Ticket" feedback form on the Help Center page.
+# ZAMMAD_FORM_ENABLED=false
+# Support e-mail address displayed on the Help Center page.
+# SUPPORT_EMAIL=support@example.com