Update documentation for Slack integration

To way we need to generate Slack tokens has changed. It would be helpful to update the documentation and possibly publish a Weekly Snippets Slack app.

Start by locating the documentation that explains how to generate Slack tokens and compare it with the current process. Determine whether publishing a Weekly Snippets Slack app is in scope; the work is complete when the token instructions are current and the app question has a documented decision.

Written by the indexing model from the issue text.