docs: Add documentation for outgoing webhooks.

Rewritten by some combination of @rheaparekh and @timabbott to more
clearly discuss what's actually important to users.

templates/zerver/api/outgoing-webhooks.md

@@ -0,0 +1,101 @@
+# Outgoing Webhooks
+
+Outgoing Webhooks allow you to build or set up Zulip integrations
+which are notified when certain types of messages are sent in
+Zulip. When one of those events is triggered, we'll send a HTTP POST
+payload to the webhook's configured URL.  Webhooks can be used to
+power a wide range of Zulip integrations.  For example, the
+[Zulip botserver][zulip-botserver] is built on top of this API.
+
+Zulip supports outgoing webhooks both in a clean native Zulip format,
+as well as a format that's compatible with
+[Slack's outgoing webhook API][slack-outgoing-webhook], which can help
+with porting an existing Slack integration to work with Zulip.
+
+[zulip-botserver]: https://zulipchat.com/api/deploying-bots#zulip-botserver
+[slack-outgoing-webhook]: https://api.slack.com/custom-integrations/outgoing-webhooks
+
+To register an outgoing webhook:
+
+* Log in to the Zulip server.
+* Navigate to *Settings (<i class="fa fa-cog"></i>)* -> *Your bots* ->
+  *Add a new bot*.  Select *Outgoing webhook* for bot type, the URL
+  you'd like Zulip to post to as the **Endpoint URL**, the format you
+  want, and click on *Create bot*. to submit the form/
+* Your new bot user will appear in the *Active bots* panel, which you
+  can use to edit the bot's settings.
+
+## Triggering
+
+There are currently two ways to trigger an outgoing webhook:
+1.  **@-mention** the bot user in a stream.  If the bot replies, its
+    reply will be sent to that stream and topic.
+2.  **Send a private message** with the bot as one of the recipients.
+    If the bot replies, its reply will be sent to that thread.
+
+## Zulip message format
+
+The Zulip-format webhook messages post the following data, encoded as JSON:
+
+```
+data:       content of message in a ready-to-use format (with the mention stripped)
+token:      A string of alphanumeric characters you can use to
+            authenticate the webhook request (each bot user uses a fixed token).
+message:    the message which triggered outgoing webhook
+├── id
+├── sender_email
+├── sender_full_name
+├── sender_short_name
+├── sender_realm_str
+├── sender_id
+├── type
+├── display_recipient
+├── recipient_id
+├── subject
+├── timestamp
+├── avatar_url
+├── client
+```
+
+A correctly implemented endpoint will do the following:
+
+* For a successful request, it should return receives either a json
+  encoded dictionary, describing how to respond to the user.
+* If the dictionary contains `response_not_required` set to `True`, no
+  response message is sent to the user.
+* If the dictionary contains `response_string` key, the corresponding
+  value is used as the `content` for a Zulip message sent in response;
+  otherwise, a generic default response message is sent.
+* For a failed request, the endpoint should return data on the
+  error.
+
+## Slack-format webhook format
+
+This interface translates the Zulip's outgoing webhook's request into
+Slack's outgoing webhook request.  Hence the outgoing webhook bot
+would be able to post data to URLs which support Slack's outgoing
+webhooks.  Here's how we fill in the fields that a Slack format
+webhook expects:
+
+```
+token:          string of alphanumeric characters (similar to above)
+team_id:        string id of the Zulip organization
+team_domain:    domain of the Zulip organization
+channel_id:     stream id
+channel_name:   stream name
+timestamp:      timestamp when message was sent
+user_id:        id of user who sent the message
+user_name:      full name of sender
+text:           content of message in a more readable format
+trigger_word:   trigger method
+service_id:     id of bot user
+```
+
+The above data is posted as list of tuples (not JSON).
+
+* For successful request, if data is returned, it returns that data,
+  else it returns a blank response.
+* For failed request, it returns the reason of failure, as returned by
+  the server, or the exception message.
+
+

templates/zerver/api/sidebar.md

@@ -14,6 +14,7 @@
 * [Running bots](/api/running-bots)
 * [Deploying bots](/api/deploying-bots)
 * [Writing bots](/api/writing-bots)
+* [Outgoing webhooks](/api/outgoing-webhooks)
 
 ## REST API