docs: add full-stack monorepo tutorial (Node frontend + Deno backend)

[bartlomieju]

Adds a tutorial under examples/tutorials that walks through a monorepo with a
Vite + React frontend running on Node, a Deno HTTP backend, and a shared package
of TypeScript code that both sides import by name through a single Deno
workspace.

The existing workspaces docs cover the building blocks (mixing deno.json and
package.json members, npm/pnpm compatibility, catalogs), but never assemble
them into the cross-runtime frontend/backend setup that people keep asking for.
This fills that gap with an end-to-end worked example: root workspace config,
a shared package, the Deno backend, the Vite frontend (using
@deno/vite-plugin so Vite resolves the workspace member), and deno check
type-checking all three members together.

I built and ran the project while writing this: the backend serves the API using
the shared code, the Vite dev server proxies /api to it, vite build bundles
the shared package, and deno check passes across the whole workspace.

Closes #1714.