build: compile the docs without Turbopack's persistent cache (#2695)

## Context

- Refs #2684. Since it merged, every preview that contains it has failed
`docs (Preview)` at `site.browser.spec.ts:258`:
`/docs/concepts/flows-and-schemas` puts its h1 at x=462.5 instead of
310.5 (seen on #2558 and #2676).

## Summary

- Next 16.4 turns `experimental.turbopackFileSystemCacheForBuild` on by
default, so `next build` reuses Turbopack's cache in `.next/cache`.
Vercel restores that folder from an earlier deployment, and Turbopack
served `app/global.css` from before #2684. The failing preview's CSS
chunk matched main's production chunk except for the missing
`#fd-spacious-layout:not(:has(…))` rule. Turbo's own cache wasn't
involved: it reported a miss and ran `next build`.
- `apps/docs/next.config.mjs` turns that cache off, so every docs build
compiles from source. The cost is a somewhat slower `next build` on
Vercel.

## Validation

- Redeployed #2558's commit `470317b49` without the build cache (`POST
/v13/deployments?forceNew=1`). The log shows "Skipping build cache", the
deployed CSS has the rule, and `docs (Preview)` passed (run
41756864792).
- `node -e "import('./next.config.mjs')…"` resolves
`experimental.turbopackFileSystemCacheForBuild: false` through
`withStory(withMDX(...))`.
- This PR's own `docs (Preview)` covers the setting on Vercel.

## Release notes / changeset

- No changeset required — no shipped behavior changed.

## Notes

- `apps/website` also runs Next 16.4 on Vercel with the same default and
could serve stale output the same way. Left out to keep this change to
the docs.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: fforootd

apps/docs/next.config.mjs

@@ -62,6 +62,13 @@ const config = {
   // service worker the package ships beside itself
   // (app/docs/mock-service-worker.js).
   serverExternalPackages: ["@zitadel/api-mock"],
+  experimental: {
+    // Next 16.4 keeps Turbopack's build cache in .next/cache, which Vercel
+    // restores from an earlier deployment; a preview built from it served a
+    // stale app/global.css that lacked a rule its source had. Every build
+    // compiles from source.
+    turbopackFileSystemCacheForBuild: false,
+  },
   // Adds each prerendered page's enforced policy, with its inline
   // scripts' hashes, once the build has written the page.
   adapterPath: fileURLToPath(new URL("./csp-adapter.mjs", import.meta.url)),