Add diagram of image-based provisioning with finish

[Lennonka]

What changes are you introducing?

Adding a sequence diagram of image-based provisioning with Finish script. Using PlantUML to build the diagram.

Why are you introducing these changes? (Explanation, links to references, issues, etc.)

Preparation for improvements of Provisioning hosts. The diagram will be used later to explain the workflow of the provisioning method.

Anything else to add? (Considerations, potential downsides, alternative solutions you have explored, etc.)

• Some files (.iuml, .pstyle) are expected to be reused in mutiple diagrams.
• Puppet steps are not currenly displayed in the resulting image. Perhaps they should be for Foreman?
• Continuation of https://github.com/theforeman/foreman-documentation/pull/3069

Checklists

• [x] I am okay with my commits getting squashed when you merge this PR.
• [x] I am familiar with the contributing guidelines.

Please cherry-pick my commits into: N/A