feat: support i18n_subsites and fix sitemap format

[diegofd]

Add i18n_subsites support for proper hreflang generation

Problem

The sitemap plugin doesn't properly support the i18n_subsites Pelican plugin. When using i18n_subsites, each language runs as a separate Pelican instance with its own SITEURL (e.g., https://example.com for the main language and https://example.com/en for subsites). This caused several issues:

1. Missing hreflang links: Translations weren't linked because i18n_subsites doesn't populate obj.translations - each language version is processed independently
2. Incorrect URLs: The sitemap used relative URLs that didn't account for the different SITEURL per language
3. Malformed paths: Translation URLs from Pelican's native mechanism contained ../ segments that weren't normalized

Solution

This PR adds proper i18n_subsites support by:

1. Tracking language context per page: Store SITEURL and DEFAULT_LANG from each page's context during queue_page(), preserving the language-specific settings from each i18n_subsites run

2. Slug-based translation grouping: Group articles/pages by their slug across all languages. Since the same content in different languages shares the same slug, this correctly links translations even when i18n_subsites processes them separately

3. Full URL generation: Build absolute URLs using each page's own SITEURL, with proper normalization of ../ path segments using posixpath.normpath

4. Dual translation mechanism: Support both:

   • Slug-based grouping (for i18n_subsites)
   • Pelican's native obj.translations (for standard multi-language setups)

Changes

• Updated XML_URL and XML_TRANSLATION templates to use full URLs
• Added get_full_url() helper with URL path normalization
• Added add_to_url_map() to build URL map with slug tracking
• Added link_translations() to merge translations from both mechanisms
• Added format_hreflang() to generate hreflang XML links
• Track current_lang and current_siteurl per queued page

Example output

Before (missing hreflang):

<url>
<loc>https://example.com/en/about/</loc>
</url>

After (proper bidirectional hreflang):

<url>
<loc>https://example.com/en/about/</loc>
<xhtml:link rel="alternate" hreflang="en" href="https://example.com/en/about/"/>
<xhtml:link rel="alternate" hreflang="es" href="https://example.com/about/"/>
</url>