docs: update e1-4-using-light-with-slack-and-teams.md

[habib424]

Drafted by Ship Ship 🚢 — confidence 🟢 high

What changed

Added a new subsection documenting that users can @mention Light in Slack channels (public/private) to get threaded replies, with each thread maintaining independent conversation context for multi-user collaboration. Updated the Privacy section to clarify that channel mentions are visible to everyone in the channel, while each thread maintains its own context.

Source PRs

• light#6935 — feat(slack): handle app_mention events with threaded replies

How to use this PR

1. Review the diff against the existing article — focus on accuracy and customer voice.
2. Merge if it reads well. Edit and push to this branch if it needs adjustment.
3. Close without merging if the article doesn't actually need this update.

Once we have a track record of clean drafts, we'll graduate to auto-merge with a delay buffer. For now: human-in-the-loop.