Adds the testimonials row to the collections table, PAYLOAD_PREVIEW_SECRET/
NEXT_PUBLIC_PAYLOAD_URL to the env vars section, and a new Live Preview
section covering /api/preview, the 3 Live-Preview-aware components, and
the next/headers RSC-boundary gotcha hit while building it.
Both were fully hardcoded before: every blockquote showed a static
"Merke dir:" label, and every post's "Passend dazu" card always linked
the same flagship product. Now driven by two new Posts fields —
quoteLabel (empty hides the label/icon/underline, blockquote still
renders) and relatedProduct (empty hides the card entirely) — mirroring
Products.spotlight but per-post instead of a single site-wide flag.
README's collection table updated to match today's Payload changes
(shipping-settings, the new Posts fields, trust-badges' placeholder
tokens, admin sidebar grouping) — also fixed a stale claim that `media`
isn't tenant-scoped; it already was.
Marco