Fix hreflang tags to only include translated locales

[pettinarip]

Built on top of #16804

Description

Fixes hreflang tags to only include translated locales.

High-level changes

• filters hreflang tags to only include locales with translations
• sets canonical URL to english version + index, follow metatag for untranslated pages
• sitemap only generate entries for translated content

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Implementation details

Hreflang fix in getMetadata()

• filters hreflang tags to only include locales with translations
• sets canonical URL to english version for untranslated pages

Refactor src/lib/utils/translations.ts

• made to get all intl translated pages easily (used in the sitemap generation)
• refactored from hardcoded if-statements to data-driven approach

Unified translation registry (src/lib/i18n/translationRegistry.ts)

• created getTranslatedLocales() to check translation status for both MD and intl pages
• single source of truth for translation status used by both metadata and sitemap

Sitemap refactor (app/sitemap.ts)

• removed old next-sitemap plugin to use a the builtin sitemap.ts file
• uses getAllPagesWithTranslations() from translation registry
• only includes URLs for pages that are actually translated in each locale
• ensures sitemap and page metadata use the same translation detection logic

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The base branch was changed.

[pettinarip]

Detected a bug. We are missing the following pages from the sitemap:

• /10years
• /contributing/translation-program/translatathon/leaderboard
• /enterprise
• /ethereum-history-founder-and-ownership
• /ethereum-vs-bitcoin
• /founders
• /quizzes
• /trillion-dollar-security
• /what-is-ether
• /what-is-the-ethereum-network

These pages don’t use a namespace for translations, so they aren’t listed in the translation registry.

We’ll need a different approach to discover intl pages. Instead of pulling them from the registry, I’ll switch to detecting them by scanning the filesystem.