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/config.toml

@@ -33,6 +33,12 @@ enableGitInfo = false
     weight = 2
     [languages.es.params]
       description = "Una app local-first y descentralizada para guardar y compartir semillas tradicionales. Sin conexión, cifrada, sin cuenta."
+  [languages.pt]
+    languageName = "Português"
+    languageCode = "pt"
+    weight = 3
+    [languages.pt.params]
+      description = "Uma app local-first e descentralizada para guardar e partilhar sementes tradicionais. Sem ligação, cifrada, sem conta."
 
 [markup.goldmark.renderer]
   unsafe = false