Add: Vercel deployment tutorial

docs/vercel.md

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+## Deploying VERT on Vercel
+
+### 1. Click the Vercel Deploy Button
+
+[![Deploy with Vercel](https://vercel.com/button)](https://vercel.com/new/clone?repository-url=https://github.com/VERT-sh/VERT&project-name=VERT&repository-name=VERT)
+
+### 2. Configure build settings
+
+| Setting            | Value          |
+|-------------------|-----------------|
+| Framework Preset   | `SvelteKit`    |
+| Build Command      | `bun run build`|
+| Output Directory   | `build`        |
+| Install Command    | `bun install`  |
+
+### 3. Set environment variables
+
+| Variable                            | Example Value                   | Description |
+|-------------------------------------|---------------------------------|------------|
+| `PUB_ENV`                           | `production`                    | Application environment (required for SvelteKit static build) |
+| `PUB_HOSTNAME`                      | `vert.sh`                       | Hostname for analytics tracking |
+| `PUB_PLAUSIBLE_URL`                 | `https://plausible.example.com` | Plausible Analytics URL (leave empty to disable) |
+| `PUB_VERTD_URL`                     | `https://vertd.vert.sh`         | VERT daemon for video conversion |
+| `PUB_DISABLE_ALL_EXTERNAL_REQUESTS` | `false`                         | Set `true` to block all external requests |
+| `PUB_DISABLE_FAILURE_BLOCKS`        | `false`                         | Set `true` to prevent blocking on repeated video conversion failures |
+| `PUB_DONATION_URL`                  | `https://donations.vert.sh`     | Stripe donation URL |
+| `PUB_STRIPE_KEY`                    | ``                              | Stripe public key |
+
+### 4. Deploy
+
+Click **Deploy**. Vercel will build the project with SvelteKit using the settings above.  
+
+This method uses Vercel’s native build system and is easy to update and maintain.