docs: Batch 2 — add Related sections to all channel pages

Add consistent Related sections to 17 channel pages that had none,
linking to: Channels Overview, Pairing, Groups, Channel Routing, Security.

Add Groups and Security links to 4 channel pages (discord, slack,
telegram, whatsapp) that already had partial Related sections.

docs/channels/feishu.md

@@ -750,3 +750,11 @@ Feishu currently exposes these runtime actions:
 - `channel-info`
 - `channel-list`
 - `react` and `reactions` when reactions are enabled in config
+
+## Related
+
+- [Channels Overview](/channels) — all supported channels
+- [Pairing](/channels/pairing) — DM authentication and pairing flow
+- [Groups](/channels/groups) — group chat behavior and mention gating
+- [Channel Routing](/channels/channel-routing) — session routing for messages
+- [Security](/gateway/security) — access model and hardening