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/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(