Improve stream multiplexing overview

[salmad3]

Done Criteria

• General overview improvements.
  • Specify connections upgrades.
  • Introduce native muxers.
• Separate the current material and create a "Muxers in libp2p" section.

Why important

The current overview is effective but limited. There is an opportunity here to give more context on multiplexing, key features, and build up to the different protocols in a similar manner seen in the other overviews across the sections.

Notes

We'll eventually add diagrams.