Add Slack channels for management groups and remove S-Group references

Updates the organizational structure page with Slack channels for each management group:

• Management group: #managers, #people-managers-and-above (private)
• Director group: #directors-and-above (private)
• Functional Leaders: #functional-leaders (private)
• E-group: #e-group (public), plus private channel for confidential matters

Also removes all references to the S-Group section and updates the Senior Directors section accordingly.