Add release-notes generator script and Makefile target

[wallrj-cyberark]

Producing release notes has always been one of the most laborious parts of the cert-manager release process. This PR adds a Go tool that automates the tedious, error-prone steps — extracting the changelog from GitHub PR history, formatting contributor and maintainer lists, and maintaining the structured MDX markers the website expects — so we spend less time on formatting and more time reviewing content.

What this adds

scripts/release-notes/ — Go CLI tool

• Uses k8s.io/release/pkg/notes to collect changelog entries from merged PRs between two git revisions.
• Auto-detects the end revision: tries the release tag (v1.21.0), then the release branch (release-1.21), then falls back to master. The tool can therefore be run before the tag exists.
• Generates a new release-notes file from a template (if not already present) with placeholder sections for summary, themes, contributors, maintainers, steering committee, and changelog.
• On subsequent runs, updates only the relevant sections (contributors, maintainers, steerers, changelog), leaving hand-edited sections untouched.
• Handles legacy files (created before this tool existed) by inserting MDX markers using structural heuristics, then regenerating the changelog section.
• Quotes PR numbers and GitHub handles for spell-checker compatibility.

make/02_mod.mk — generate-release-notes target

export GITHUB_TOKEN=$(gh auth token)
make generate-release-notes        # uses cert_manager_latest_version from variables.json
make generate-release-notes CERT_MANAGER_VERSION=v1.21.0

content/docs/contributing/release-process.md — updated to document the new tool and Makefile target.

content/docs/releases/release-notes/release-notes-1.21.md — first real use of the tool: generated the v1.21.0 pre-release notes from 216 commits since v1.20.0.

Testing

Unit tests cover:

• Creating a new versioned changelog section from an unversioned placeholder
• Inserting a new version above existing versioned sections (newest-first ordering)
• Replacing an existing section for the same version (handles pre-release → final transitions)
• Legacy files: inserting community markers via structural heuristics
• Legacy files: replacing a bare ## \vX.Y.Z`` heading with generated content on first run
• Contributor extraction from changelog handles ([@handle] and [`@handle`] forms)

Manually verified by running make generate-release-notes CERT_MANAGER_VERSION=v1.21.0 against the existing release-notes-1.21.md file that was created manually — the tool inserted the community markers and generated the full changelog, including PRs merged up to the time of the run.

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[maelvls]

I've tested this script, but wasn't able to run it because I hadn't create the tag yet. The release process says that I should prepare the release notes before releasing. Here is what I'm getting:

$ cd scripts/release-notes
$ go run . --release-notes-dir ../../content/docs/releases/release-notes --release-version v1.20.1
INFO[0000] Cloning/updating repository cert-manager/cert-manager
INFO[0015] Using found start SHA: 0d2f215fe26ca2674758b5dfd780f04ac1dfde92
invalid configuration: invalid notes options: resolving v1.20.1: reference not found
exit status 1

[cert-manager-prow]

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[cert-manager-prow]

PR needs rebase.

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