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/2_customization/05_sitemap.md

@@ -0,0 +1,77 @@
+---
+title: Customizing the Sitemap
+intro: Fine-tune the built-in sitemap or replace it entirely
+---
+
+The built-in sitemap generator has a few options to adjust its behavior. For most sites, these are enough. If you need full control, you can replace the generator with your own.
+
+## Excluding templates
+
+By default, only the `error` template is excluded. To exclude more templates:
+
+```php
+<?php
+// site/config/config.php
+
+return [
+  'tobimori.seo' => [
+    'sitemap' => [
+      'excludeTemplates' => ['error', 'redirect', 'internal'],
+    ],
+  ],
+];
+```
+
+## Grouping by template
+
+By default, all pages end up in a single sitemap. If you have many pages, you can split them into separate sitemaps per template. This creates a sitemap index at `/sitemap.xml` with links to `/sitemap-blog.xml`, `/sitemap-product.xml`, etc.
+
+```php
+'sitemap' => [
+  'groupByTemplate' => true,
+],
+```
+
+## Change frequency and priority
+
+Both `changefreq` and `priority` accept a static value or a callable:
+
+```php
+'sitemap' => [
+  'changefreq' => 'daily',
+  'priority' => fn (Page $page) => $page->isHomePage() ? 1.0 : 0.5,
+],
+```
+
+The default `changefreq` is `weekly`. The default `priority` is calculated from page depth: the homepage gets `1.0`, each level deeper subtracts `0.2`, down to `0.2`.
+
+## Writing your own generator
+
+If the options above aren't enough, you can replace the entire sitemap generator. The `generator` option takes a callable that receives a `SitemapIndex` instance. Here's a minimal example:
+
+```php
+<?php
+
+use tobimori\Seo\Sitemap\SitemapIndex;
+
+return [
+  'tobimori.seo' => [
+    'sitemap' => [
+      'generator' => function (SitemapIndex $sitemap) {
+        $index = $sitemap->create('pages');
+
+        foreach (site()->index()->listed() as $page) {
+          $index->createUrl($page->url())
+            ->lastmod($page->modified())
+            ->changefreq('weekly')
+            ->priority(0.8);
+        }
+      },
+    ],
+  ],
+];
+```
+
+`$sitemap->create('key')` creates a sitemap group. `$index->createUrl($url)` adds a URL entry, and you can chain `->lastmod()`, `->changefreq()`, `->priority()`, and `->alternates()` on it.
+
+The built-in generator does more: it filters by robots settings, respects `excludeTemplates`, handles `groupByTemplate`, and adds hreflang links for multilingual sites. You can find its source in `config/options/sitemap.php` as a reference for your own.