Add diagrams of provisioning methods

[Lennonka]

New diagrams to describe provisioning methods in PlantUML (SVG export available).

The diagrams will not be included in the docs for now, but I will include them once I'll document the provisioning methods properly. This classification of provisioning methods is incompatible with the current docs.

Classification of provisioning methods

• Installer-based provisioning with network boot

  • PXE boot
  • HTTP boot

• Installer-based provisioning with boot disk

  • Full host image
  • Subnet image

• Image-based provisioning

  • with cloud-init script
  • with Finish script

In some cases I had to guess a lot, so don't take my word as gospel and raise your concerns if you think it's wrong.

• [x] I am familiar with the contributing guidelines.

Please cherry-pick my commits into: N/A (hoping to finish it for 3.12)