server-sdks: link the Next.js guide from the adapter list #55

build/server-sdks.mdx lists the server adapters but not @seamless-auth/nextjs. The Next.js adapter is documented on its own page, build/nextjs.mdx (#51), which went there to avoid conflicting with #40, a rewrite of server-sdks.mdx that is still open.

Once #40 merges (or as part of it), add @seamless-auth/nextjs to the adapter list on server-sdks.mdx with a one-line description ("Next.js App Router: serves /auth from a route handler, reads the session in server components and proxy.ts") and a link to Next.js.