docs(slack): add missing scopes for DM replies

The manifest was missing scopes required for conversations.open API,
which is used to get DM channel IDs for replies.

Added scopes:
- im:write (required for DM replies)
- im:read (list DM conversations)
- mpim:write (reply to multi-person DMs)
- mpim:read (list MPDMs)
- groups:write (private channel interactions)
- groups:read (list private channels)

Without im:write, the example config (dm.enabled: true) cannot
actually reply to DMs - fails with missing_scope error.

docs/slack.md

@@ -50,8 +50,14 @@ Use this Slack app manifest to create the app quickly (adjust the name/command i
         "channels:history",
         "channels:read",
         "groups:history",
+        "groups:read",
+        "groups:write",
         "im:history",
+        "im:read",
+        "im:write",
         "mpim:history",
+        "mpim:read",
+        "mpim:write",
         "users:read",
         "app_mentions:read",
         "reactions:read",