docs: update feature documentation for Mega-ES

Update the product documentation to reflect new functionalities and refinements:
- Add "Mega Hub" integration options for WhatsApp, Instagram, and Messenger.
- Update the base platform description and last update date.
- Detail improvements to the call permission system regarding the 24-hour window.
- Document new sidebar quick filters, reactive unread counters, and improved conversation sorting.
- Add internal mention tracking within private notes.