[i18n] Add banner to non-en pages that have drifted

[chalin]

Tracked by #4467.

Consider showing a banner on non-en pages that are out-of-date relative to their corresponding en page. It would be nice to have a link to the en page Git diff.

Tasks

• Propose a means for scripts & Hugo to know when pages have drifted - [CI] check-i18n: add support for drifted_from_default field #6444
• Tag some pages as drifted: [i18n] Add drifted_from_default status to drifted pages, adjust script, add "Drift status" section to l10n page #6446
• Add banner for drifted pages - [i18n] Display page-outdated banner for non-en pages that have drifted #6475
• Make it easier to localize the page-outdated banner
• Update CI to check for proper drifted status. (It's not clear to me how best to do this atm. Options include on every PR, though it would be nice to be able to skip running fix:i18n:status when en-page changes are just trivial fixes, ... as a maintainer could attest to by adding, say, a label to that effect. But that's getting a bit involved. Hopefully we'll find a suitable incremental path.)