docs: update references to Iris v14 and adjust example links in documentation
docs: Go 1.27 API in the migration page, struct handlers, encoding, broadcast, debugging and connections pages
docs: refresh and expand project documentation Add new conceptual documentation (Architecture, Reconnection, Choosing-a-backend, Examples) and expand existing docs (Broadcast, Errors, Installation, Namespaces, Rooms, Scale-out, Upgraders-and-dialers). Reorganize sidebar navigation.