adds docs for retentive integration

Author: muhammadatifjav

src/connections/destinations/catalog/retentive/index.md

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+[Retentive](https://retentive.io/?utm_source=segmentio&utm_medium=docs&utm_campaign=partners) makes your help docs searchable in product so your GTM team can act on data of what each customer is struggling with.
+
+This destination is maintained by Retentive. For any issues with the destination, [contact the Retentive Support team](mailto:help@retentive.io).
+
+## Getting Started
+
+{% include content/connection-modes.md %}
+
+1. From the Destinations catalog page in the Segment App, click **Add Destination**.
+2. Search for "Retentive" in the Destinations Catalog, and select the "Retentive" destination.
+3. Choose which Source should send data to the "Retentive" destination.
+4. Go to the [Retentive integrations tab](https://app.retentive.io/integrations).
+5. Toggle the Segment integration on, and copy the API key presented.
+6. Enter the "API Key" in the "Retentive" destination settings in Segment.
+
+## Supported methods
+
+Retentive supports the following methods, as specified in the [Segment Spec](/docs/connections/spec).
+
+### Identify
+
+Send [Identify](/docs/connections/spec/identify) calls to track user metadata alongside their search queries in the Retentive dashboard. For example:
+
+```js
+analytics.identify('userId123', {
+  email: 'john.doe@example.com'
+});
+```
+
+Customer data will only be shown once they perform searches on Retentive.
+
+### Alias
+
+Send [Alias](/docs/connections/spec/alias) calls to merge customers from different destinations into a single customer in Retentive. For example:
+
+```js
+analytics.alias(
+    'primaryId',
+    'previousId' // optional
+);
+```