Improve demo video playback experience

The demo video playback had a number of issues:

- Looping was confusing as it didn't make it clear when the video had
  ended.

- Playback was too fast. It was hard to follow some of the more complex
  examples at the default playback speed. Slowing this down makes it
  easier for all viewers to understand.

pages/before-you-start.md

@@ -92,6 +92,6 @@ video {
 }
 </style>
 
-<video autoplay controls loop>
+<video autoplay controls onloadstart="this.playbackRate = 0.67;">
   <source src="/videos/before-you-start.webm" type="video/webm">
 </video>

pages/do-the-change.md

@@ -50,7 +50,7 @@ layout: full
 preload: false
 ---
 
-<video autoplay controls loop>
+<video autoplay controls onloadstart="this.playbackRate = 0.67;">
   <source src="/videos/do-the-change-feature1.webm" type="video/webm">
 </video>
 
@@ -95,7 +95,7 @@ layout: full
 preload: false
 ---
 
-<video autoplay controls loop>
+<video autoplay controls onloadstart="this.playbackRate = 0.67;">
   <source src="/videos/do-the-change-feature2.webm" type="video/webm">
 </video>
 
@@ -223,6 +223,6 @@ layout: full
 preload: false
 ---
 
-<video autoplay controls loop>
+<video autoplay controls onloadstart="this.playbackRate = 0.67;">
   <source src="/videos/do-the-change-new-commits.webm" type="video/webm">
 </video>

pages/fix-your-mistakes.md

@@ -37,7 +37,7 @@ layout: full
 preload: false
 ---
 
-<video autoplay controls loop>
+<video autoplay controls onloadstart="this.playbackRate = 0.67;">
   <source src="/videos/fix-your-mistakes-amend.webm" type="video/webm">
 </video>
 
@@ -62,7 +62,7 @@ layout: full
 preload: false
 ---
 
-<video autoplay controls loop>
+<video autoplay controls onloadstart="this.playbackRate = 0.67;">
   <source src="/videos/fix-your-mistakes-restore.webm" type="video/webm">
 </video>
 
@@ -96,6 +96,6 @@ layout: full
 preload: false
 ---
 
-<video autoplay controls loop>
+<video autoplay controls onloadstart="this.playbackRate = 0.67;">
   <source src="/videos/fix-your-mistakes-commit.webm" type="video/webm">
 </video>

pages/prepare-the-commit.md

@@ -57,7 +57,7 @@ layout: full
 preload: false
 ---
 
-<video autoplay controls loop>
+<video autoplay controls onloadstart="this.playbackRate = 0.67;">
   <source src="/videos/prepare-the-commit-squash.webm" type="video/webm">
 </video>
 
@@ -128,7 +128,7 @@ layout: full
 preload: false
 ---
 
-<video autoplay controls loop>
+<video autoplay controls onloadstart="this.playbackRate = 0.67;">
   <source src="/videos/prepare-the-commit-update-message.webm" type="video/webm">
 </video>