[Docs] Document Polls (#2930)

* poll docs

* oopsie

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

@@ -0,0 +1,4 @@
+// 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();