feat(site): add Portuguese to the landing site

Mirror the es/ pattern: docs/legal/pt/ + docs/o-que-e-tane.md as the
Portuguese source of truth, build-legal.sh extended to generate the
Hugo pt.* pages from them, config.toml gets a [languages.pt] entry,
i18n/pt.json translates the UI chrome. og-pt.png is a placeholder copy
of og-en.png until a proper Portuguese social card is made by hand
(same as the other og-*.png, per DEPLOY.md). Verified locally with the
Docker build: /pt/, /pt/about/, all four /pt/legal/ pages and the
lang-switch link all resolve correctly.

site/content/legal/_index.pt.md

@@ -0,0 +1,4 @@
+---
+title: "Legal"
+description: "Política de privacidade, termos de uso, regras da comunidade e aviso sobre a legalidade das sementes do Tane."
+---