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.