www-gitlab-com/data syncs break main (missing YAML schema validation)

Summary

The script in scripts/sync-data.sh downloads YAML files from the data/ directory in www-gitlab-com. This is required for

1. Rendering tables and overviews in the handbook
2. Local Codeowner checks

Whenever a change in www-gitlab-com is merged, the latest data is pulled

Relevant logs and/or screenshots and/or links to examples

!5495 (merged) caused by gitlab-com/www-gitlab-com@0960b7c9

Error: error building site: render: failed to render pages: render of "section" failed: "/builds/gitlab-com/content-sites/handbook/layouts/all-content/list.html:36:7": execute of template failed: template: all-content/list.html:36:7: executing "all-content/list.html" at <.Content>: error calling Content: failed to render shortcode: "/builds/gitlab-com/content-sites/handbook/content/handbook/engineering/infrastructure/performance-indicators.md:5:1": failed to render shortcode "performance-indicators": failed to process shortcode: "/builds/gitlab-com/content-sites/handbook/layouts/shortcodes/performance-indicators.md:41:16": execute of template failed: template: shortcodes/performance-indicators.md:41:16: executing "shortcodes/performance-indicators.md" at <partial "performance-indicators/health-reasons" .>: error calling partial: "/builds/gitlab-com/content-sites/handbook/layouts/partials/performance-indicators/health-reasons.html:3:16": execute of template failed: template: partials/performance-indicators/health-reasons.html:3:16: executing "partials/performance-indicators/health-reasons.html" at <plainify>: error calling plainify: unable to cast map[string]interface {}{"Cause for broken `master` for April 2024":"flaky tests (45%), infrastructure/runner issues (42%), job timing out (17%), various infrastructure issues (11%), failed to pull job image (9%), runner disk full (5%), merge train missing (3%), test gap (3%), dependency upgrade (3%), broken ci config (2%), GitLab.com overloaded (2%)"} of type map[string]interface {} to string

Possible solutions

1. Add basic YAML schema validation into handbook sync script, and bail out early with helpful error messages.
2. Add YAML validation early in www-gitlab-com and prevent MRs from being merged.
3. Move everything into a dedicated repository, and implement guardrails and schema validation, proposed in #171 (closed)

Notes

1. The data sync script lives in public, internal, and docsy theme Git repository, and needs updates eventually. Validation in www-gitlab-com should be preferred therefore.
2. Migration from www-gitlab-com to another repository needs stakeholder review, dependency updates, etc. and can block the boring solution of stopping in case of errors. Prefer a solution that does not allow to merge broken YAML code.