Emit BCP 47 hreflang tags for new locales

The hreflang attribute requires BCP 47 language tags, but the existing
template was emitting the raw locale key (e.g. 'zh_hans') as the
hreflang value. Underscores aren't valid in BCP 47, so the new locales
would have produced non-conformant alternate links that search engines
ignore.

Adds an optional 'hreflang' key to each locale's data file, with the
template falling back to the locale key when the key is absent. That
keeps every existing locale's output unchanged (single-subtag codes
like 'es', 'pt', 'de' are already valid BCP 47) and lets zh_hans and
zh_hant emit zh-Hans and zh-Hant.

Addresses Copilot review feedback on PR #1333.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Andrea Griffiths

_data/locales/zh_hans.yml

@@ -1,5 +1,6 @@
 zh_hans:
   locale_name: 简体中文
+  hreflang: zh-Hans
   OpenSourceFriday: "开源星期五"
   full_description: "众人拾柴火焰高。这个星期五，为你在用的、喜欢的项目做出贡献。"
   Contributors: "贡献者"

_data/locales/zh_hant.yml

@@ -1,5 +1,6 @@
 zh_hant:
   locale_name: 繁體中文
+  hreflang: zh-Hant
   OpenSourceFriday: "開源星期五"
   full_description: "眾人拾柴火焰高。這個星期五，為你在用的、喜歡的項目做出貢獻。"
   Contributors: "貢獻者"

_includes/head.html

@@ -11,13 +11,15 @@
   {% assign locales = site.data.locales | sort %}
   {% for locale in locales %}
   {% assign lang = locale[0] %}
+  {% assign lang_data = locale[1][lang] %}
+  {% assign hreflang_tag = lang_data.hreflang | default: lang %}
   {% assign page_lang_slash = page.lang | append: '/' | prepend: '/' %}
   {% assign default_url = page.url | replace: page_lang_slash, '/' %}
   {% if lang == "en" %}
   <link rel="alternate" hreflang="en" href="{{ site.url }}{{ default_url }}" />
   <link rel="alternate" hreflang="x-default" href="{{ site.url }}{{ default_url }}" />
   {% else %}
-  <link rel="alternate" hreflang="{{ lang }}" href="{{ site.url }}/{{ lang }}{{ default_url }}" />
+  <link rel="alternate" hreflang="{{ hreflang_tag }}" href="{{ site.url }}/{{ lang }}{{ default_url }}" />
   {% endif %}
   {% endfor %}
   {% endif %}