Document the guidelines for group-pings

In order to enable people to effectively utilize their `@channel` permissions, we should document the guidelines for when channel pings are appropriate.

IMO this should include:
- who is allowed to have access to this permission (i.e. working group leaders)
- what the process is for this permission to be granted (automatic? by request only)
- Which channels is this permission allowed to be used in (limit based on total number of members? channel age? avoid specific large channels like #general and #random?)
- What types of announcements are generally considered "worth" pinging the channel for
- When these pings should be sent (i.e. be careful you dont ping someone at an odd time because its near the end of the day in your timezone)
- how frequently are pings like this allowed to be used (to prevent annoyance)
- What is the process for raising a concern about any of the above items if a ping recipient does not like, or wants to opt-out from receiving these pings? (there should be a way to do so without forcing them to stop participating in the working group)

No file or test is named. Start by locating the repository’s existing infrastructure documentation and any current guidance on channel permissions, then resolve the open policy questions with project stakeholders. Done means a published guideline covers eligibility, granting access, permitted channels, appropriate timing and frequency, announcement scope, and recipient concerns or opt-out handling.

Written by the indexing model from the issue text.