Add diagram of installer-based provisioning with HTTP boot

[Lennonka]

What changes are you introducing?

Adding a sequence diagram of installer-based provisioning with HTTP boot. 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.
• 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