Populate mplex stream muxer doc

## Done Criteria
- [x] Create a "What is mplex" section
- [x] Create a "How does mplex work" section
- [x] Create a "mplex in libp2p" section
  - Note usage of mplex and yamux and should not need to support mplex, unless dealing with legacy nodes

## Why Important
Page was created as a placeholder and requires additional material.