docs: flip README links to docs.picpeak.app + delete docs/_to-migrate (#1000 phase 3) (#1023)

Phase 3 (final) of #1000. The deep content now lives on the docs site (PicPeak/docs#7), making docs.picpeak.app the single source of truth and removing the in-repo copies.

README links flip to docs.picpeak.app; the roadmap table is retired in favour of GitHub Issues. Deletes docs/_to-migrate/ and the five migrated pages. docs/migration-to-org.md stays — it's repo-transitional, not docs-site content.

In-app references to the deleted files are repointed at the docs site, including the CRM disclaimer strings in en.json/de.json and the contract-editor fallback.

Closes #1000.

backend/src/routes/adminFeatureFlags.js

@@ -63,7 +63,7 @@ const KNOWN_FLAGS = [
   // are a standalone legal document type with their own composition
   // (blocks) and signing flow (in-browser canvas + wet-signed PDF
   // upload). Seeded block bodies are EXAMPLES ONLY; admins must have a
-  // lawyer review before sending. See docs/crm-disclaimers.md.
+  // lawyer review before sending. See https://docs.picpeak.app/features/crm/disclaimers.
   'contracts',
   // Accounting (migration 122). Top-level Accounting area — inbound
   // supplier invoices, expenses + re-bill, and the tax report (which