docs: clarify Telegram group chat troubleshooting

[LeonSGP43]

Summary

• add a Telegram group troubleshooting checklist for bots that respond in DMs but stay silent in groups
• call out BotFather privacy delivery, remove/re-add behavior, Hermes sender/chat allowlists, and mention filters
• clarify that negative group chat IDs belong in TELEGRAM_GROUP_ALLOWED_CHATS

Fixes #18580

Test plan

• git diff --check origin/main...HEAD
• docs-only change; no runtime tests run

[LeonSGP43]

CI triage: docs-site-checks is failing in Lint docs diagrams on website/docs/user-guide/features/skills.md lines 248/254-256/261. This PR only changes website/docs/user-guide/messaging/telegram.md, and the changed file passes the same lint locally with ascii-guard 2.3.0 (Errors: 0). Running the full docs lint locally reproduces the same pre-existing table false positive outside this PR scope, so I am leaving the Telegram docs branch focused on #18580.