docs: set up Shibuya multilingual translation scaffolding

- Set language = "en" and add the Shibuya navbar "Translations" switcher
  via html_context["languages"]: English, then Simplified Chinese
  (zh_CN / zh-Hans), then Traditional Chinese (zh_TW / zh-Hant), with the
  gettext + sphinx-intl + per-language build workflow documented inline.
- Remove the stale locale/zh_CN translations -- they predate all the new
  tutorial content and would mistranslate against it. Fresh .po files get
  generated from the current docs at translation time.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: kwagyeman

docs/conf.py

@@ -316,14 +316,35 @@ def _render_landing_code(src):
 # breakdown, so use the same version identifier for both to avoid confusion.
 version = release = micropython_version
 
-# The language for content autogenerated by Sphinx. Refer to documentation
-# for a list of supported languages.
-# language = None
+# The source language. Translations are built by overriding this on the
+# command line, e.g. ``-D language=zh_CN``.
+language = "en"
 
 # -- Options for i18n output ----------------------------------------------
 locale_dirs = ["locale"]
 gettext_compact = False
 
+# Shibuya's navbar "Translations" switcher and the <link rel="alternate"
+# hreflang> tags read this list. Each entry is
+#   (display name, URL pattern with %s for the page, hreflang).
+# English is the source language at the site root; each translation is a
+# separate per-language build deployed under its own subdirectory. To
+# produce one:
+#
+#   pip install sphinx-intl
+#   sphinx-build -b gettext . _build/gettext        # extract .pot
+#   sphinx-intl update -p _build/gettext -l zh_CN -l zh_TW
+#   # ...translate locale/<lang>/LC_MESSAGES/*.po...
+#   sphinx-build -b html -D language=zh_CN . _build/html/zh_CN
+#   sphinx-build -b html -D language=zh_TW . _build/html/zh_TW
+#
+# The zh_CN / zh_TW entries below go live once those builds are deployed.
+html_context["languages"] = [
+    ("English", "/%s.html", "en"),
+    ("简体中文", "/zh_CN/%s.html", "zh-Hans"),
+    ("繁體中文", "/zh_TW/%s.html", "zh-Hant"),
+]
+
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used:
 # today = ''