Refactor for multiple deployment targets

[ES-Alexander]

• Versioned BlueOS docs will be hosted at blueos.cloud/docs (via BlueOS-deploy branch (WIP))
• Versioned ArduSub docs will be hosted at ardusub.com (via a new ArduSub-deploy branch perhaps?)
  • Requires completion of content transfer from the existing ardusub-gitbook repo, per
    • [WIP] Docs transfer #32
    • ArduSub 4.0 transfer from gitbook #33
    • QGroundControl 4.0 transfer #34
    • ArduSub 4.1 transfer #40
    • etc
• In future Cockpit docs will be hosted at blueos.cloud/cockpit/docs
• Docs for a specific version of BlueOS will be made available as a BlueOS extension (where the extension version corresponds to the version of BlueOS it documents)
• ArduSub docs will also be made available locally in BlueOS, likely via some form of extension
  • Not sure yet if it's best to provide all versions, or just a specific version per extension version like for BlueOS
  • Not sure yet how best to handle docs for other ArduPilot firmwares
    • Should they be bundled with the ArduSub ones on a per-version basis?
    • Should they be separated by vehicle type?
    • Should there be a single "ArduPilot Docs Manager" extension that allows selecting firmware types and versions to download / remove docs for?
      • This seems likely to be the best user experience, but is likely also extra development effort

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• Some of the docs (e.g. hardware/integration) are shared amongst multiple targets, so should be refactored into their own branches and pulled in as appropriate via submodules (to avoid needing to maintain them separately)
• We may also want to refactor the zola templates into a submodule for the same reason