Adding docs for Segment RETL destination (#3911)

* Adding docs for Segment RETL destination

* First edits

Going to make more edits, but committing these for now

* Update Segment destination docs for public beta

* Update index.md

* Update to the steps for connecting the Segment destination

Updated the steps for connecting and configuring the Segment destination. The goal of this doc update is to prevent customers from accidentally only sending a full sync to the first mapping created. It is therefore recommended to configure and enable all mappings before enabling the Segment destination.

* Update destination ID

* Set to private

Co-authored-by: Sai Gattu <sai.gattu@segment.com>
Co-authored-by: kdaswani <49517136+kdaswani@users.noreply.github.com>
Co-authored-by: James Reynolds <jameshreynolds@gmail.com>
Co-authored-by: markzegarelli <mark.zegarelli@segment.com>

Author: 5 people

src/connections/destinations/catalog/actions-segment/index.md

@@ -0,0 +1,38 @@
+---
+title: Segment Destination
+hide-boilerplate: true
+hide-dossier: false
+id: 6371eee1ae5e324869aa8b1b
+private: true
+---
+
+The Segment destination enables you to mold data extracted from your warehouse into [Segment Spec](/docs/connections/spec/) API calls that can be processed by Segment's Tracking API.
+
+**The Segment destination can be connected to Reverse ETL warehouse sources only.**
+
+> info ""
+> The Segment (Actions) destination is in beta and is in active development. Some functionality may change before it becomes generally available.
+
+## Getting started
+
+### Create a source
+1. From the Segment web app, navigate to **Sources** and select **Add Source**.
+2. Search for **HTTP API** in the Sources Catalog, select the HTTP API source, and click **Add Source**.
+3. Follow the steps to name your source. This is the source that will receive API calls from the Segment destination.
+4. Copy the Write Key on the **Overview** tab. You will need this when you set up the Segment destination.
+
+### Connect and configure the Segment destination
+1. From the Segment web app, navigate to **Reverse ETL > Destinations**.
+2. Click **Add Destination** in top-right corner.
+3. Select the Segment destination, click **Next**, and select the warehouse source that will send data to the Segment destination. If you have not set up a warehouse source, follow the steps in the Reverse ETL documentation on [Getting started](/docs/reverse-etl/#getting-started).
+4. On the **Settings** tab, name your destination, input the Write Key from the source created above, select an endpoint region, and click **Save Changes**. It is recommended to configure and enable all mappings before enabling the Segment destination.
+5. On the **Mappings** tab, click **Add Mapping**. Select a data model and the API call type you want to map. Fill in the fields on screen to create the desired mappings, and click **Create Mapping** to complete the configuration. Repeat this step to configure multiple mappings. 
+6. Enable the configured mapping(s).
+7. On the **Settings** tab, click the **Enable Destination** toggle, and then click **Save Changes** to enable the Segment destination.
+
+{% include components/actions-fields.html settings="true"%}
+
+## FAQ & Troubleshooting
+
+### API Calls and MTUs
+The Segment destination sends data to Segment's Tracking API, which has cost implications. New users will count as new MTUs and each call will count as an API call. For information on how Segment calculates MTUs and API calls, please see [MTUs, Throughput and Billing](/docs/guides/usage-and-billing/mtus-and-throughput/).