[Docs] Document Polls (#2930)

* poll docs

* oopsie

docs/guides/polls/samples/create-poll.cs

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+
+// Create a poll
+var poll = new PollProperties
+{
+    // Set the question
+    Question = new ()
+    {   // Text of the question
+        Text = "Discord.Net is awesome!"
+    },
+    // Set the duration of the poll in hours
+    Duration = 69,
+    // Add answers to the poll
+    // You can add from 1 to 10 answers
+    Answers = [
+        // An answer can consist of text and an emoji
+        new PollMediaProperties
+        {   // Text for the answer
+            Text = "Yes!",
+            // Emoji for the answer
+            // Can be a custom emoji or unicode one
+            // Remember that bot must be in the guild where the custom emoji is
+            Emoji = Emote.Parse("<:wires:1214532316999974962>")
+        },
+        // Or just text
+        new PollMediaProperties
+        {
+            Text = "Of course!",
+        }
+        ],
+    // You can allow users to select multiple answers
+    // By default, it's set to false
+    AllowMultiselect = true,
+    // Also you can set the layout of the poll
+    // By default, it's set to Default
+    // At this time, it's the only available layout type
+    LayoutType = PollLayout.Default
+};
+
+// Send the poll to the text channel
+await textChannel.SendMessageAsync(poll: poll);

docs/guides/polls/samples/end-poll.cs

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+// Get a message with a poll
+var message = await channel.GetMessageAsync(id) as IUserMessage;
+// End the poll
+await message.EndPollAsync();

docs/guides/polls/samples/get-poll-voters.cs

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+// Get the id of the first answer in the poll
+var answerId = message.Poll.Answers.First().AnswerId;
+// Get the list of voters who voted for the first answer
+var voters = await message.GetPollAnswerVotersAsync(answerId).FlattenAsync();