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 Add diagrams of provisioning methods #3069

Checklists

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

Please cherry-pick my commits into: N/A

[github-actions]

The PR preview for 562fcca is available at theforeman-foreman-documentation-preview-pr-3305.surge.sh

The following output files are affected by this PR:

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[Lennonka]

Updated based on feedback from #3304

[Lennonka]

We should tweak the diagram in such way so that it doesn't contain TFTP. Foreman uses the TFTP folder to do some steps, but HTTP boot doesn't use the TFTP protocol.

[Lennonka]

I've made an attempt to tweak the diagram so that it uses Proxy instead of TFTP as a participant, but I'm not sure if it's technically correct. We'll discuss tomorrow during our session.

[stejskalleos]

In the #3304 PR, there is a TFTP participant. Should we add the participant HTTP here as well?

[Lennonka]

@stejskalleos I believe that we discussed last week that we will not introduce an HTTP participant but treat the Foreman Proxy as the server that provides HTTP.

[stejskalleos]

ACK.

[stejskalleos]

🍏 LGTM