docs: clarify changelog fragment workflow

[vyncint]

Description

Clarifies the relationship between changelog fragments and the root CHANGELOG.md for contributors. This change documents:

• when a changelog fragment is expected and when pr/no-changelog is appropriate;
• how to add a PR-numbered fragment after opening a pull request;
• how release tooling collects fragments and generates PR links;
• the complete set of documented changelog categories; and
• a changelog checkpoint in the pull request template.

Closes #32030.

Changelog

• This change only updates repository contributor documentation, so it does not need a changelog fragment. Please apply the pr/no-changelog label.

Testing

• git diff HEAD^ --check
• Verified the documented filename and label behavior against .github/workflows/changelog-checker.yml.

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.
• Not applicable: this documentation-only change does not alter security controls.
• Not applicable: this documentation-only change does not require a special revert plan.

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[Copilot]

Pull request overview

This PR clarifies the contributor-facing workflow for changelog fragments and how those fragments relate to the root CHANGELOG.md, including when pr/no-changelog is appropriate and adding a changelog checkpoint to the PR template.

Changes:

• Expanded CONTRIBUTING.md guidance on when to add a changelog fragment, expected categories, and why CHANGELOG.md should not be edited directly.
• Added contributor-focused workflow documentation to changelog/README.md, including notes about fragment collection into CHANGELOG.md.
• Updated the PR template to include a changelog checklist item and a pointer to the changelog guidance.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
CONTRIBUTING.md	Documents when/why to add changelog fragments, categories, and how fragments feed CHANGELOG.md.
changelog/README.md	Adds a “Contributor workflow” section explaining fragment expectations and release aggregation.
.github/pull_request_template.md	Adds a changelog checklist item linking to the contributor guidance.