📝 Add banner to translated pages (#14809)

* Add banner to translated pages

* Add link to English version. Use modern syntax for details block

* 🎨 Auto format

* Move `translation-banner.md` inside `docs` directory

* 🎨 Auto format

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

scripts/mkdocs_hooks.py

@@ -26,6 +26,17 @@ def get_missing_translation_content(docs_dir: str) -> str:
     return missing_translation_path.read_text(encoding="utf-8")
 
 
+@lru_cache
+def get_translation_banner_content(docs_dir: str) -> str:
+    docs_dir_path = Path(docs_dir)
+    translation_banner_path = docs_dir_path / "translation-banner.md"
+    if not translation_banner_path.is_file():
+        translation_banner_path = (
+            docs_dir_path.parent.parent / "en" / "docs" / "translation-banner.md"
+        )
+    return translation_banner_path.read_text(encoding="utf-8")
+
+
 @lru_cache
 def get_mkdocs_material_langs() -> list[str]:
     material_path = Path(material.__file__).parent
@@ -151,4 +162,21 @@ def on_page_markdown(
         if markdown.startswith("#"):
             header, _, body = markdown.partition("\n\n")
         return f"{header}\n\n{missing_translation_content}\n\n{body}"
-    return markdown
+
+    docs_dir_path = Path(config.docs_dir)
+    en_docs_dir_path = docs_dir_path.parent.parent / "en/docs"
+
+    if docs_dir_path == en_docs_dir_path:
+        return markdown
+
+    # For translated pages add translation banner
+    translation_banner_content = get_translation_banner_content(config.docs_dir)
+    en_url = "https://fastapi.tiangolo.com/" + page.url.lstrip("/")
+    translation_banner_content = translation_banner_content.replace(
+        "ENGLISH_VERSION_URL", en_url
+    )
+    header = ""
+    body = markdown
+    if markdown.startswith("#"):
+        header, _, body = markdown.partition("\n\n")
+    return f"{header}\n\n{translation_banner_content}\n\n{body}"