Replace apropos/licence/charte sticky nav with responsive details/summary TOC

- Convert .apropos-toc <nav> to <details class="toc" open> in all three templates
- Add caret-down icon to summary (visible only on mobile via media queries)
- Desktop (≥768px): sticky sidebar via CSS grid, force-open via pointer-events:none, hide caret
- Mobile (≤767px): single column, collapsible TOC with rotating caret, margin-top below search bar
- Rename .apropos-toc-link to .toc-sidebar-link
- Merge 900px and 600px breakpoints into single 600px mobile media query
- Rename apropos.css → content-page.css; update 3 controller references

app/src/Controllers/LicenceController.php

@@ -49,7 +49,7 @@ class LicenceController
             'pageTitle'        => $pageTitle . ' – XAMXAM',
             'metaDescription'  => "Informations sur les licences d'utilisation des mémoires publiés sur XAMXAM, le répertoire des TFE de l'erg.",
             'currentNav'       => 'licence',
-            'extraCss'         => ['/assets/css/apropos.css'],
+            'extraCss'         => ['/assets/css/content-page.css'],
             'bodyClass'        => 'apropos-body',
         ];
     }