[FIX] website_slides: fix embed-over-nav overlap

The `ratio` class used for embeds creates a static context.
Using `position: relative` for the subsequent element makes sure it doesn't
end up 'under' the embed on large viewports sizes.

For simplicity, we chose to not test for the actual presence of an embed
or window size before applying position-relative on the next div.

Task-2663320
Part of odoo/odoo#79615

Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>

addons/website_slides/views/website_slides_templates_lesson.xml

@@ -309,7 +309,7 @@
         </div>
     </div>
 
-    <div class="mb-5">
+    <div class="mb-5 position-relative">
         <ul class="nav nav-tabs o_wslides_lesson_nav" role="tablist">
             <li class="nav-item">
                 <a href="#about" aria-controls="about" t-att-class="'nav-link active' if not comments else 'nav-link'" role="tab" data-bs-toggle="tab">