docs: Add MNTN Audiences destination documentation

src/connections/destinations/catalog/actions-mntn-audiences/index.md

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+---
+title: MNTN Audiences Destination
+---
+
+{% include content/plan-grid.md name="actions" %}
+
+[MNTN](https://mountain.com/?utm_source=segmentio&utm_medium=docs&utm_campaign=partners){:target="_blank"} is a Connected TV advertising platform that helps performance marketers reach targeted audiences and measure campaign results across streaming TV.
+
+This destination is maintained by MNTN. For any issues with the destination, [contact their Support team](mailto:support@mountain.com).
+
+## Getting started
+
+1. From your workspace's [Destination catalog page](https://app.segment.com/goto-my-workspace/destinations/catalog){:target="_blank"} search for "MNTN Audiences".
+2. Select **MNTN Audiences** and click **Add Destination**.
+3. Select an existing Engage Audience as the Source to connect to MNTN Audiences.
+4. Log in to the [MNTN Integrations Marketplace](https://mountain.com/integrations){:target="_blank"} and generate an **API Key** for the Segment integration.
+5. Enter the **API Key** in the MNTN Audiences destination settings in Segment.
+6. Connect the destination to an Engage Audience. MNTN will automatically create a new audience segment in your MNTN account when the connection is first enabled. If you want to sync to a pre-existing MNTN segment instead, enter its **Segment ID** in the audience-level settings.
+
+{% include components/actions-fields.html %}
+
+## Additional context
+
+### Audience segment creation
+
+When an Engage audience is connected to this destination, Segment calls MNTN's Audience API to create a new audience segment automatically and stores the returned segment ID. That segment ID is then used for all subsequent Sync Audience calls — no manual configuration is required.
+
+If you already have a MNTN segment you want to sync to, enter its ID in the **MNTN Segment ID** field under the audience-level settings when configuring the destination. This bypasses automatic creation and uses your existing segment instead.
+
+### Identity signals
+
+The Sync Audience action sends all available identity signals to MNTN to maximize audience match rates. Signals are read from event traits (for identify calls) or properties (for track calls).
+
+| Signal | Field | Notes |
+|--------|-------|-------|
+| Email | `traits.email` or `context.traits.email` | Lowercased before sending. Sent as both plaintext and SHA-256 hash. |
+| Phone | `traits.phone` or `properties.phone` | All non-numeric characters (including `+`) are stripped automatically before sending. Sent as both plaintext and SHA-256 hash. |
+| IP Address | `traits.ip` or `properties.ip` | Sent as plaintext IPv4. |
+| Mobile Advertising ID | `traits.advertisingId` or `properties.advertisingId` | IDFA on iOS or GAID on Android. |
+
+At least one identity signal should be present for effective audience matching. Providing more signals increases match rates in MNTN campaigns.
+
+### Adding and removing users
+
+A single **Sync Audience** action handles both adding and removing users from a MNTN segment. Segment Engage passes the audience membership status with each event — users are added to the segment when they enter the audience and removed when they exit. No separate configuration is needed to handle both cases.