docs: document npm 12 install script blocking (#9838)

## Summary

Add the omitted npm 12 breaking-change note explaining that dependency
lifecycle scripts are blocked by default unless covered by
`allowScripts`, including the approval and rebuild workflow.

## Cause

The change was introduced in
[`5cd5150`](https://github.com/npm/cli/commit/5cd5150d3e85dcf5d246e7e5c9de216c2ff849db).
Although its message described a v12-only default flip, it used `feat:`
instead of `feat!:` and did not include a `BREAKING CHANGE:` footer.
Release Please therefore classified it as a regular feature and omitted
it from the aggregated npm 12 breaking-change notes.

## Release notes

The published [`v12.0.0` GitHub
release](https://github.com/npm/cli/releases/tag/v12.0.0) was corrected
manually with the same breaking-change entry. This PR corrects the
source-controlled changelog used by the npm documentation site.

## Manual correction process

If a breaking change is omitted from release notes in the future:

1. Do not rewrite the merged commit. Add the missing entry under the
released version’s `⚠️ BREAKING CHANGES` section in the root
`CHANGELOG.md` and submit a documentation PR.
2. After the PR merges, the npm documentation repository’s scheduled
**Update CLI** workflow copies the root changelog into the corresponding
CLI documentation page and publishes it. Dispatch that workflow manually
if the docs need to update immediately.
3. Update the existing GitHub release separately because a changelog PR
cannot modify an already-published release. Preserve the complete
current release body before editing it because `gh release edit
--notes-file` replaces the entire body:

   ```bash
gh release view <tag> --repo npm/cli --json body --jq .body > release.md
   # Add the same breaking-change entry to release.md.
   gh release edit <tag> --repo npm/cli --notes-file release.md
   ```

4. Verify that the source changelog, npm documentation page, and GitHub
release contain identical wording.

To prevent the omission, breaking commits must use a conventional-commit
breaking marker such as `feat!:` and include a `BREAKING CHANGE:` footer
describing the user-visible impact.

Fixes #9750