feat: intégration plugin Kirby SEO

- Ajout de tobimori/kirby-seo via Composer
- snippet('seo/head') dans header.php (remplace les meta manuels)
- snippet('seo/schemas') dans footer.php pour JSON-LD
- Onglet SEO ajouté dans site.yml et tous les blueprints de pages
- Configuration SEO dans config.php (sitemap, robots, canonicalBase TODO)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

site/plugins/kirby-seo/docs/1_features/01_sitemap.md

@@ -0,0 +1,35 @@
+---
+title: Sitemap
+intro: A sitemap for search engines, generated from your pages
+---
+
+Kirby SEO generates an XML sitemap at `https://example.com/sitemap.xml`. Search engines like Google use it to discover all the pages on your site. You don't need to create or update it manually: it updates whenever your content changes.
+
+What you see here are the defaults. The sitemap generator and all its options can be changed or replaced entirely. See [Customizing the sitemap](2_customization/05_sitemap) for details.
+
+## What's in the sitemap
+
+The sitemap only includes pages that are [visible to search engines](1_features/00_robots-indexing). Unlisted pages, drafts, and pages excluded by robots settings are left out. The `error` template is also excluded by default.
+
+Each page in the sitemap includes:
+
+- `loc`: the page URL
+- `lastmod`: when the page was last modified
+- `changefreq`: how often the page is likely to change (default: `weekly`)
+- `priority`: how important the page is relative to other pages on your site
+
+Priority is calculated from page depth: the homepage gets `1.0`, and each level deeper subtracts `0.2`, down to a minimum of `0.2`.
+
+A `Sitemap:` line is also added to your [robots.txt](1_features/00_robots-indexing) automatically, so crawlers know where to find it.
+
+## Multilingual sites
+
+If your Kirby site has multiple languages, the sitemap automatically includes `hreflang` links for each page. These tell search engines which language versions of a page exist, so they can show the right one in search results.
+
+Only languages where a translation actually exists are included. There is no separate sitemap per language: all translations are listed in a single sitemap using `<xhtml:link>` elements.
+
+## Browser view
+
+If you open `https://example.com/sitemap.xml` in a browser, you'll see a styled table instead of raw XML. This is powered by an XSL stylesheet that Kirby SEO serves at `/sitemap.xsl`. On multilingual sites, each URL shows language badges linking to its alternate translations.
+
+To see the raw XML, use `view-source:https://example.com/sitemap.xml` in your browser's address bar.