Update mender-docs to align it with new onboarding

In the new UI onboarding, the focus is for the user to use real devices potentially with no full dual rootfs integration. Our mender-docs tutorial still focuses on the virtual device and real devices with the demo images.

We want to keep them as aligned as possible.

Acceptance criteria:

• The mender-docs tutorial follows closer the new onboarding flow:
  • "Install the mender server" should estate that client is an optional parameter
  • "Deploy to physical devices" should go before the virtual ones, clarifying the difference between a demo integration and a full dual rootfs integration.
• "Deploy to virtual devices" page is removed.
• We keep the rpi/beagle demo images for now, but encourage the install of mender via the .deb package.
• The difference between full integration and use for application updates only (which is the case for onboarding) is outlined. More information of full integration is provided.