[Feature] Expose GetCurrentUser method & missing current user API methods (#2574)

* add stuff

* forgot to push

* docs

* Update DiscordRestClient.cs

* Apply suggestions from code review

* Update toc.yml

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Co-authored-by: Casmir <68127614+csmir@users.noreply.github.com>

src/Discord.Net.Core/Entities/Guilds/IUserGuild.cs

@@ -18,5 +18,13 @@ namespace Discord
         ///     Returns the current user's permissions for this guild.
         /// </summary>
         GuildPermissions Permissions { get; }
+
+        /// <summary>
+        ///     Gets the features for this guild.
+        /// </summary>
+        /// <returns>
+        ///     A flags enum containing all the features for the guild.
+        /// </returns>
+        GuildFeatures Features { get; }
     }
 }