WIP: feat(mnq): split mnq into three products

[RoRoJ]

Feature branch for M&Q split.

This feature branch removes all M&Q doc. Branches to be merged into this one will add doc for NATS, Topics and Events, and Queues.

Before merging this branch to main:

• Put redirections in place
• Sort out changelog filters
• Sort out tutorial filters
• Check all internal links within the doc are updated
• Update -categories metadata in doc pages as necessary