Migrate to Hugo v0.146.0+ new template system

[EricGusmao]

Fixes #2785

Migrates Blowfish to Hugo's new template system introduced in v0.146.0. See the Hugo new template system overview for details.

Summary

• Restructures layouts/ to comply with Hugo v0.146.0+'s new template system: removes _default/, renames partials/ → _partials/, shortcodes/ → _shortcodes/, and promotes render hooks to _markup/
• Renames index.html → home.html, terms.html → taxonomy.html, and index.json → home.json to match new Hugo page-kind template naming
• Replaces deprecated {{ template "_internal/opengraph.html" }} / {{ template "_internal/twitter_cards.html" }} calls with {{ partial "..." }}, and updates all templates.Exists / printf paths from "partials/..." to "_partials/..."
• Applies the same _partials/ / _shortcodes/ renames to exampleSite/layouts/
• Bumps minimum Hugo version in config.toml to 0.146.0

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[nunocoracao]

Thanks for working on this! Unfortunately, this PR introduces breaking changes for existing users — anyone who overrides partials (e.g. layouts/partials/extend-footer.html, layouts/partials/extend-head.html, layouts/partials/favicons.html, etc.) would have their customizations silently stop working after upgrading. We don't accept breaking changes unless they're strictly required to keep supporting the project (e.g. a dependency deprecating a feature we rely on). Hugo's new template system with _partials is supported but the old partials path still works, so this migration isn't required yet. If Hugo ever removes support for the old paths, we'd revisit this. Thanks for understanding!