docs: Add missing documentation for required tests and branch protection

[Copilot]

Problem

The architecture documentation was missing an explanation of what controls the "Required" tests that appear in GitHub PRs. Users were confused about:

• What creates these required status checks
• Why some tests appear as "Required" but cannot be triggered with /test
• The relationship between Prow jobs and GitHub's branch protection

This gap was highlighted in issue #XXXXX where a user specifically asked: "Looking at the architecture document, I do not understand what controls the contents of this block in a PR on GitHub. What adds required tests? Those do not come from ProwJobs, right?"

Solution

Added a new "Required Tests and Branch Protection" section to the architecture documentation that explains:

• Required tests are GitHub branch protection rules, not Prow jobs themselves
• How Prow's branchprotector component can automatically configure these settings
• The relationship between always_run: true Prow jobs and required status checks
• Common sources of "mysterious" required tests that users cannot /test

Also added a cross-reference from the branchprotector documentation back to the architecture overview to help users understand the bigger picture.

Key clarifications provided

1. Required tests ≠ Prow jobs - They are GitHub's enforcement of status checks
2. Three common sources of required tests:
   • Explicit branch-protection configuration
   • Automatically derived from always_run: true Prow jobs
   • Manually set in GitHub's branch protection settings
3. Clear path to solution - Links to branchprotector documentation for configuration details

This addresses the documentation gap identified by maintainers and provides the missing context that users need to understand Prow's integration with GitHub branch protection.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Missing documentation on required tests</issue_title>
<issue_description>Looking at the architecture document https://docs.prow.k8s.io/docs/overview/architecture/, I do not understand what controls the contents of this block in a PR on GitHub.

What adds required tests? Those do not come from ProwJobs, right? I have a mysterious one (pull-kubestellar-kubestellar-validate-prow-yaml) that I do not know where it is coming from (and I can not /test it).

</issue_description>

Comments on the Issue (you are @copilot in this section)

@petr-muller The items shown as `Required` are configured through GitHub's [branch protection](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/about-protected-branches) concept. That can be configured through Prow's [branchprotector](https://docs.prow.k8s.io/docs/components/optional/branchprotector/) component. The applied settings can be either origin from the explicit config, or it can be derived from Prowjob configuration - non-optional, [unconditionally triggered](https://docs.prow.k8s.io/docs/jobs/#triggering-jobs) jobs. @petr-muller Two of the links I provided lead to Prow documentation, one leads to the documentation for the component that controls the GH settings you asked about. Obviously the docs can always get better.

Fixes #451

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[k8s-ci-robot]

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[k8s-ci-robot]

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Copilot
Once this PR has been reviewed and has the lgtm label, please ask for approval from petr-muller. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment