feature: Add missing properties to Guild and deprecate GuildEmbed (#1573)

* Add missing properties to Guild, related methods, and deprecate GuildEmbed endpoints

- Add missing guild properties: `discovery_splash`, `widget_enabled`, `widget_channel_id`, `rules_channel_id`, `max_presences`, `max_presences`, `max_members`, `public_updates_channel_id`, `max_video_channel_users`, `approximate_member_count`, `approximate_presence_count`
- Update guild properties: `embed_enabled`, `embed_channel_id`
- Add `GetGuildDiscoverySplashUrl` to `CDN`
- Add classes related to the guild widget
- Add `withCounts` parameter to `GetGuild(s)Async`
- Make GuildEmbed related methods obsolete with a message redirecting to widget ones

* Change xml docs and PremiumSubscriptionCount type

* Changed some xml docs

src/Discord.Net.Core/CDN.cs

@@ -73,11 +73,21 @@ namespace Discord
         /// <param name="guildId">The guild snowflake identifier.</param>
         /// <param name="splashId">The splash icon identifier.</param>
         /// <returns>
-        ///     A URL pointing to the guild's icon.
+        ///     A URL pointing to the guild's splash.
         /// </returns>
         public static string GetGuildSplashUrl(ulong guildId, string splashId)
             => splashId != null ? $"{DiscordConfig.CDNUrl}splashes/{guildId}/{splashId}.jpg" : null;
         /// <summary>
+        ///     Returns a guild discovery splash URL.
+        /// </summary>
+        /// <param name="guildId">The guild snowflake identifier.</param>
+        /// <param name="discoverySplashId">The discovery splash icon identifier.</param>
+        /// <returns>
+        ///     A URL pointing to the guild's discovery splash.
+        /// </returns>
+        public static string GetGuildDiscoverySplashUrl(ulong guildId, string discoverySplashId)
+            => discoverySplashId != null ? $"{DiscordConfig.CDNUrl}discovery-splashes/{guildId}/{discoverySplashId}.jpg" : null;
+        /// <summary>
         ///     Returns a channel icon URL.
         /// </summary>
         /// <param name="channelId">The channel snowflake identifier.</param>