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/Dockerfile

@@ -68,7 +68,7 @@ RUN rm -rf /usr/local/lib/node_modules/npm /usr/local/bin/npm /usr/local/bin/npx
 # PDFs to flat PNGs server-side so the admin UI NEVER renders a raw (possibly
 # malicious) PDF. pdftoppm does not execute embedded JS or fetch remote
 # resources, so it doubles as the SSRF/phone-home guard for untrusted inbound
-# documents (see docs/accounting-inbound-invoices.md).
+# documents (see https://docs.picpeak.app/features/accounting/incoming-invoices).
 # exiftool extracts the embedded full-res JPEG preview from RAW/DNG uploads
 # (Apple ProRAW etc.) — sharp's libvips has no raw loader, so the pipeline
 # thumbnails/displays that preview while keeping the original for download.

backend/server.js

@@ -497,7 +497,7 @@ app.use('/uploads', setCorsHeaders, secureStatic(path.join(storagePath, 'uploads
 // are stable (e.g. Inter/400.woff2), so an admin replacing the file on disk
 // must be able to roll out the change to clients. With max-age + Last-Modified
 // (set by express.static from file mtime), browsers send If-Modified-Since
-// after expiry and pick up the new version automatically. See docs/fonts.md
+// after expiry and pick up the new version automatically. See https://docs.picpeak.app/guides/custom-fonts
 // "Replacing an existing font" for the documented rollout strategy.
 const fontStaticOpts = { maxAge: '7d' };
 app.use(

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