Rebranding "Broker" to "Realtime"

### Description

I've been playing [Supabase](https://supabase.com/) recently for a personal project. It is a Postresql wrapper, but also has an offering for a pub/sub model. They brand this as "Realtime", and I really like that as a term for our own MQTT offering, it covers the scope of "UNS" and "MQTT", without being industry/UNS-specific and would cover anything else we want to introduce here.

The term is a lot more user friendly, and clearer on the _value_ of the offering, not just the technical component involved.

**Task:** Change the left-side navigation option to "Realtime", and update the page description to be more onboarding/new-user friendly

### Epic/Story

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate