desktop/multi-arch: various (markdown) touch-ups
desktop/multi-arch: various (markdown) touch-ups

- remove mention of "experimental" for buildx
- use "console" where possible
- removed use of Bash "heredoc" in example, which may not work on
  shells other than Bash
- wrap content to ~80 chars
- enable TOC on page

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

thaJeztah/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io