Add Talk OCP docs to dev manual

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

developer_manual/digging_deeper/index.rst

@@ -21,6 +21,7 @@ Digging deeper
    rest_apis
    search
    settings
+   talk
    two-factor-provider
    users
    dashboard

developer_manual/digging_deeper/talk.rst

@@ -0,0 +1,63 @@
+================
+Talk Integration
+================
+
+`Nextcloud Talk <https://apps.nextcloud.com/apps/spreed>`_ is the chat, video and audio conferencing solution in Nextcloud. Apps can access Talk capabilities through a public API.
+
+
+All communication from an app to the Talk backend is handled through a *broker* ``\OCP\Talk\IBroker``. :ref:`Inject <dependency-injection>` the Talk broker into your class to make use of it.
+
+
+Check for Talk existence
+------------------------
+
+Talk is an optional app. It has to be installed and enable to be used.
+
+.. code-block:: php
+
+    <?php
+
+    /** @var \OCP\Talk\IBroker $broker */
+    if ($broker->hasBackend()) {
+        // Do something with it
+    } else {
+        // Hide Talk integration from a user or use other communication channels, if applicable
+    }
+
+
+Create a conversation
+---------------------
+
+A new conversation needs a name and at least one moderator. By default this conversation will be private.
+
+.. code-block:: php
+
+    <?php
+
+    /** @var \OCP\Talk\IBroker $broker */
+    /** @var \OCP\IUser $alice */
+    /** @var \OCP\IUser $bob */
+    $conversation = $broker->createConversation(
+        'Weekly 1:1',
+        [$alice, $bob]
+    );
+
+
+Customize the conversation
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It's possible to adjust the defaults.
+
+.. code-block:: php
+
+    <?php
+
+    /** @var \OCP\Talk\IBroker $broker */
+    /** @var \OCP\IUser $alice */
+    /** @var \OCP\IUser $bob */
+    $options = $broker->newConversationOptions();
+    $conversation = $broker->createConversation(
+        'Weekly 1:1',
+        [$alice, $bob],
+        $broker->newConversationOptions()->setPublic()
+    );