Infrastructure changes for new Mender docs sites

This may need to be broken down into smaller tasks.

• Move current docs to an 'archive' site
• New docs Grav site to replace production and staging
  • Site content to sync with public content repo using 2 way git-sync plugin https://github.com/trilbymedia/grav-plugin-git-sync
  • Site code (Grav, plugins and themes) should sync with a different private repo, using our own one-way syncing
• New separate API docs Slate site to be set up at docs.mender.io/api and synced one-way from a new private repo
  • Also likely needs a staging site
• Anything else?