Adding initial User Behavior Insights documentation.

Signed-off-by: jzonthemtn <[email protected]>

_search-plugins/user-behavior-insights/index.md

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+---
+layout: default
+title: User Behavior Insights
+parent: Search relevance
+has_children: false
+nav_order: 220
+---
+
+# User behavior insights
+
+User Behavior Insights, or UBI, is a community plugin for capturing client-side events and queries for the purposes of improving search relevance and user experience. UBI consists of three components:
+
+* An OpenSearch [plugin](https://github.com/o19s/opensearch-ubi) that facilitates the storage of client-side events and queries.
+* A client-side JavaScript library that demonstrates how to capture events and send those events to the OpenSearch UBI plugin.
+* An OpenSearch Dashboards plugin that provides access to the collected events and queries for analysis. This component is still largely in the design phase.
+
+UBI is currently only supported in OpenSearch 2.12.0.
+{: .warning }
+
+## Installing the plugin
+
+The User Behavior Insights plugin is available for OpenSearch 2.12.0. Run the following command to install the plugin.
+
+````bash
+./bin/opensearch-plugin install \
+   "https://github.com/o19s/opensearch-ubi/releases/download/0.0.8/opensearch-ubi-0.0.8-os2.12.0.zip"
+````
+
+After installing the UBI plugin you can find [documentation](https://github.com/o19s/opensearch-ubi/blob/main/documentation.md).
+
+## Using the plugin
+
+The following steps outline how to get started with the UBI plugin.
+
+### Initializing a UBI store
+
+A "store" contains the client-side events and the queries. After installing the plugin we need to initialize a store. In the following request we are creating a store called `mystore`.
+
+```
+PUT _plugins/ubi/mystore
+```
+
+A store consists of two OpenSearch indexes whose names are based on the name of the store being created. For the `mystore` store, the names of the indexes are:
+
+* `.mystore_events` - This index contains the client-side events sent to the plugin by the JavaScript library.
+* `.mystore_queries` - This index contains the queries that the plugin passively stores.
+
+### Sending events to the store
+
+With the store initialized, we can now send client-side events to our `mystore` store.
+
+```
+POST /_plugins/ubi/mystore -H "Content-Type: application/json" -d '
+{
+  "type": "instant-search",
+  "keywords": "laptop",
+  "url": "my_url.html",
+  "lang": "en-US",
+  "session": "npckcy4",
+  "channel": "demo-channel"
+}'
+```
+
+### Capturing queries
+
+The UBI plugin passively stores queries. To illustrate this, we can run a query against our index of products called `ecommerce`.
+
+`GET /ecommerce/_search -H "X-ubi-store: awesome"`
+
+Note that this request includes a header called `X-ubi-store`. This header allows the UBI plugin to associate this query with a UBI store.