docs(transitions): add poster for videos

Author: atinux

docs/content/1.getting-started/5.transitions.md

@@ -55,7 +55,7 @@ To start adding transition between your pages, add the following CSS to your [`a
 
 This produces the following result when navigating between pages:
 
-<video controls class="rounded">
+<video controls class="rounded" poster="https://res.cloudinary.com/nuxt/video/upload/v1665061349/nuxt3/nuxt3-page-transitions_umwvmh.jpg">
   <source src="https://res.cloudinary.com/nuxt/video/upload/v1665061349/nuxt3/nuxt3-page-transitions_umwvmh.mp4" type="video/mp4">
 </video>
 
@@ -96,7 +96,7 @@ definePageMeta({
 
 Moving to the about page will add the 3d rotation effect:
 
-<video controls class="rounded">
+<video controls class="rounded" poster="https://res.cloudinary.com/nuxt/video/upload/v1665063233/nuxt3/nuxt3-page-transitions-cutom.jpg">
   <source src="https://res.cloudinary.com/nuxt/video/upload/v1665063233/nuxt3/nuxt3-page-transitions-cutom.mp4" type="video/mp4">
 </video>
 
@@ -187,7 +187,7 @@ definePageMeta({
 
 This produces the following result when navigating between pages:
 
-<video controls class="rounded">
+<video controls class="rounded" poster="https://res.cloudinary.com/nuxt/video/upload/v1665065289/nuxt3/nuxt3-layouts-transitions_c9hwlx.jpg">
   <source src="https://res.cloudinary.com/nuxt/video/upload/v1665065289/nuxt3/nuxt3-layouts-transitions_c9hwlx.mp4" type="video/mp4">
 </video>
 
@@ -360,7 +360,7 @@ const next = computed(() => '/' + (id.value + 1))
 
 The page now applies the `slide-left` transition when going to the next id and `slide-right` for the previous:
 
-<video controls class="rounded">
+<video controls class="rounded" poster="https://res.cloudinary.com/nuxt/video/upload/v1665069410/nuxt3/nuxt-dynamic-page-transitions.jpg">
   <source src="https://res.cloudinary.com/nuxt/video/upload/v1665069410/nuxt3/nuxt-dynamic-page-transitions.mp4" type="video/mp4">
 </video>