Complete Dutch (nl) translations

[lwekuiper]

Completes the Dutch (nl) translation, which was previously partial.

What changed

• messages.php — filled the missing keys; now covers all 141 keys present in en/messages.php.
• fieldsets/content.php, defaults.php, sections.php — filled the missing keys (the newer json_ld*, robots*, og_* and twitter_* fields were untranslated); each now matches its en counterpart key-for-key.

Register: informal (je)

The existing nl strings were a mix of formal (u/uw) and informal (je). I standardized on informal, to match the convention used across the ecosystem:

• Statamic core (statamic/cms) nl — predominantly informal (je).
• Laravel-Lang (the canonical Laravel nl source, aggregating Jetstream/Fortify/Cashier/Nova/Spark) — informal.

Small fixes included

• Fixed a mistyped rules key unique_titlepage_failing → unique_title_page_failing (the original never resolved).
• Removed a stale image key in messages.php that no longer exists in en.
• Translated two strings that had been left in English (search_section, section canonical_url label).
• Minor typo/casing fixes ("weergeven" → "weergegeven", sentence-cased the robots toggle labels).

settings.php was already complete and is left unchanged. Files pass pint with the repo config.

[duncanmcclean]

Bedankt!