Switch to the new GHCR Docker image
Switch to the new GHCR Docker image

as the officially recommended one.

Also simplify and clean up installation
instructions. Reformat all code fragments
to a more standard Markdown.

puppet-community/puppetboard,johnzimm/puppetboard,johnzimm/xx-puppetboard,voxpupuli/puppetboard,johnzimm/xx-puppetboard,voxpupuli/puppetboard,johnzimm/puppetboard,puppet-community/puppetboard,voxpupuli/puppetboard,johnzimm/xx-puppetboard,johnzimm/xx-puppetboard,johnzimm/puppetboard,puppet-community/puppetboard