fix: add all platforms to webhook cross-platform delivery

The delivery tuple in webhook.py only had 5 of 14 platforms with
gateway adapters. Adds whatsapp, matrix, mattermost, homeassistant,
email, dingtalk, feishu, wecom, and bluebubbles so webhooks can
deliver to any connected platform.

Updates docs delivery options table to list all platforms.

Follow-up to cherry-picked fix from olafthiele (PR #7035).

gateway/platforms/webhook.py

@@ -186,14 +186,22 @@ class WebhookAdapter(BasePlatformAdapter):
         if deliver_type == "github_comment":
             return await self._deliver_github_comment(content, delivery)
 
-        # Cross-platform delivery (telegram, discord, etc.)
+        # Cross-platform delivery — any platform with a gateway adapter
         if self.gateway_runner and deliver_type in (
             "telegram",
             "discord",
             "slack",
             "signal",
             "sms",
+            "whatsapp",
+            "matrix",
             "mattermost",
+            "homeassistant",
+            "email",
+            "dingtalk",
+            "feishu",
+            "wecom",
+            "bluebubbles",
         ):
             return await self._deliver_cross_platform(
                 deliver_type, content, delivery