Add hreflang logic and consolidate canonical partial

What does this MR do and why?

Implements hreflang tags for Japanese localization SEO and consolidates URL generation logic into a single partial.

Problem: Without hreflang tags, search engines can't understand language relationships between English and Japanese pages, potentially causing duplicate content penalties.

Solution:

1. Detects when Japanese pages use English fallback content
2. Generates appropriate hreflang tags based on translation availability
3. Consolidates canonical and hreflang logic for maintainability

Hreflang Behavior

Fully translated pages:

<link rel="alternate" hreflang="en" href="https://docs.gitlab.com/page-path/" />
<link rel="alternate" hreflang="ja-jp" href="https://docs.gitlab.com/ja-jp/page-path/" />
<link rel="alternate" hreflang="x-default" href="https://docs.gitlab.com/page-path/" />

English fallback content:

<link rel="alternate" hreflang="en" href="https://docs.gitlab.com/page-path/" />
<link rel="alternate" hreflang="x-default" href="https://docs.gitlab.com/page-path/" />
<!-- No ja-jp hreflang since no translation exists -->

Changes

1. Renamed canonical-url.html → canonical-hreflang.html
2. Single loop processes all translations (performance improvement)
3. Unified URL generation using consistent base URL
4. Proper fallback content detection

Closes gitlab-com/localization/docs-site-localization#313 (closed)

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment.
2. Enable japanese site, languages.ja-jp.disabled: false in config/_default/hugo.yaml
3. make view
4. Visit ja-jp and en-us versions of page with full japanese translations. Verify correct canonical and hreflang.
   • Example: http://localhost:1313/ja-jp/topics/build_your_application/
5. Visit ja-jp and en-us versions of page that utilized english fallback content on the Japanese version. Verify correct canonical and hreflang.
   • Example: http://localhost:1313/ja-jp/ci/inputs/

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.