Update alerts documentation for webhooks (#333)

Update the alerts documentation with some information about configuring
webhook alerts.

---------

Co-authored-by: Tom Faulhaber <github1_com@infolace.com>

Author: f-wright

docs/docs/guide/9-alerts.md

@@ -24,4 +24,48 @@ Consider configuring a "no queries submitted" alert to monitor system health. If
 
 ## Alert Mediums
 
-Groundlight supports the following alerts via Email, Text Message (SMS), and Webhooks.
+Groundlight supports alerts via Email, Text Message (SMS), and Webhooks.
+
+### Webhooks
+
+By setting up a webhook alert for your Groundlight detector, you can integrate Groundlight's computer vision technology with your existing messaging platform, other tech stack, or even a custom application.
+
+You can either set up an alert using one of our default templates (currently available for Slack, more platforms to come) or build your own.
+
+#### Custom Templates
+
+We use Jinja2 to manage and render templates. See the ([Jinja template documentation](https://jinja.palletsprojects.com/en/stable/templates/)) for information on how to construct your template.
+The template will need to be a valid Jinja template which renders to valid JSON to be used as a payload for your webhook alert. 
+
+We provide a set of variables which you can use to put information about your detector and alert into your template. 
+The available variables are:
+
+- `detector_name`: The name of the detector that the alert was triggered on
+- `detector_query`: The detector's query.
+- `detector_id`: The detector's unique ID
+- `confidence_threshold`: The current confidence threshold for the detector.
+- `detector_mode`: The detector's mode (binary, count, multiclass, etc). Currently, alerts are only available for binary detectors.
+- `image_query_id`: The id of the image query which triggered the alert
+- `time_repr`: A human readable string of the time the alert was triggered in UTC. Does not include the date.
+- `activation_time`: The time and date the alert was triggered in UTC.
+- `condition_repr`: The condition the alert is configured with, put into a human-readable string (eg. " returned a YES answer at ").
+- `image_url`: An image URL to access the image which triggered the alert. Only available if the alert was configured with `include_image` set to True.
+
+For example, the template below could be used as a custom payload to configure a Slack alert which includes a basic message and the triggering image.
+```
+"""{
+    "text": "Alert activated on Groundlight detector {{ detector_name }} (Query: {{ detector_query }}). Detector {{ condition_repr }} {{ time_repr }}",
+    "attachments": [
+        {
+            "fallback": "Optional image attachment for the detector alert.",
+            "image_url": "{{ image_url }}",
+            "title": "Related Image"
+        }
+    ]
+}"""
+```
+
+#### Headers
+
+Optionally, you can also configure the headers for your webhook alert POST request. This is particularly useful if your application requires a specific security token to be present to accept incoming POST requests. 
+If your application requires headers, you can provide them as a JSON dictionary. If not, you can configure your template and leave the headers blank.