feat: Added new docs

docusaurus/docs/Flutter/stream_chat/pinned_messages.mdx

@@ -4,3 +4,61 @@ sidebar_position: 22
 title: Pinned Messages
 ---
 
+Pinned messages allow users to highlight important messages, make announcements, or temporarily promote content.
+Pinning a message is, by default, restricted to certain user roles, but this is flexible.
+Each channel can have multiple pinned messages and these can be created or updated with or without an expiration.
+
+### Pin and unpin a message
+
+An existing message can be updated to be pinned or unpinned by using the `channel.pinMessage` and `channel.unpinMessage` methods.
+Or a new message can be pinned when it is sent by setting the `pinned` and `pin_expires` fields when using `channel.sendMessage`.
+
+```dart
+// create pinned message
+final message = await channel
+    .sendMessage(Message(
+      text: 'my message',
+      pinned: true,
+      pinExpires: DateTime.now().add(Duration(days: 3)),
+    ))
+    .then((resp) => resp.message);
+
+// unpin message
+await channel.unpinMessage(message);
+
+// pin message for 120 seconds
+await channel.pinMessage(message, 120);
+
+// change message expiration to 2077
+await channel.pinMessage(message, DateTime(2077));
+
+// remove expiration date from pinned message
+await channel.pinMessage(message, null);
+```
+
+To pin the message user has to have PinMessage permission.
+You can find the list of permissions and defaults in Permission Resources and Default Permissions sections
+
+### Retrieve pinned messages
+
+You can easily retrieve the last 10 pinned messages from the `channel.pinned_messages` field:
+
+```dart
+// get channel state
+final channelState = await channel.query();
+
+// get pinned messages from it
+final pinnedMessages = channelState.pinnedMessages;
+```
+
+To learn more about channels you can visit Querying Channels page
+Search for all pinned messages
+
+Stream Chat also provides search filter in case if you need to display more than 10 pinned messages in specific channel.
+
+```dart
+// list all pinned messages of the channel
+final response = await channel.search(
+    messageFilters: Filter.equal('pinned', true),
+);
+```