docs: update getting-started thumbnail and alt text

packages/docs/docs/codelabs/index.mdx

@@ -15,7 +15,7 @@ Blockly Codelabs provide a guided, tutorial, hands-on coding experience of Block
     title="Getting started with Blockly"
     href="/codelabs/getting-started/codelab-overview">
     <Image
-      alt="Blockly icon"
+      alt="Loop block with 'Add text' block that takes the string 'Blockly' as an input" 
       src="/images/codelabs/card_thumbnails_getting-started.png"
       srcDark="/images/codelabs/card_thumbnails_getting-started-dark.png"
     />
@@ -25,7 +25,7 @@ Blockly Codelabs provide a guided, tutorial, hands-on coding experience of Block
     title="Customizing a Blockly toolbox"
     href="/codelabs/custom-toolbox/codelab-overview">
     <Image
-      alt="Blockly icon"
+      alt="A list of different colour categories labeled 'Logic', 'Loops', 'Math', and 'Text' with the label 'Custom Toolbox'"
       src="/images/codelabs/card_thumbnails_blockly-toolbox.png"
       srcDark="/images/codelabs/card_thumbnails_blockly-toolbox-dark.png"
     />
@@ -35,7 +35,7 @@ Blockly Codelabs provide a guided, tutorial, hands-on coding experience of Block
     title="Customizing your themes"
     href="/codelabs/theme-extension-identifier/codelab-overview">
     <Image
-      alt="Blockly icon"
+      alt="An addition block with shadow blocks and labels pointing out block colour, shadow block colour, and border colour"
       src="/images/codelabs/card_thumbnails_themes.png"
       srcDark="/images/codelabs/card_thumbnails_themes-dark.png"
     />
@@ -45,7 +45,7 @@ Blockly Codelabs provide a guided, tutorial, hands-on coding experience of Block
     title="Customizing context menus"
     href="/codelabs/context-menu-option/codelab-overview">
     <Image
-      alt="Blockly icon"
+      alt="An if do block with a context menu that displays options 'Duplicate', 'Add Comment', and 'Inline Inputs'"
       src="/images/codelabs/card_thumbnails_context-menus.png"
       srcDark="/images/codelabs/card_thumbnails_context-menus-dark.png"
     />
@@ -55,7 +55,7 @@ Blockly Codelabs provide a guided, tutorial, hands-on coding experience of Block
     title="Block validation and warnings"
     href="/codelabs/validation-and-warnings/codelab-overview">
     <Image
-      alt="Blockly icon"
+      alt="Two list blocks: one is attached to a print block and creates a list of numbers 0 to 5, another creates a list with numbers from 6 to 3 and is grayed out with a warning icon"
       src="/images/codelabs/card_thumbnails_validating-and-displaying.png"
       srcDark="/images/codelabs/card_thumbnails_validating-and-displaying-dark.png"
     />
@@ -65,7 +65,7 @@ Blockly Codelabs provide a guided, tutorial, hands-on coding experience of Block
     title="Use CSS in Blockly"
     href="/codelabs/css/codelab-overview">
     <Image
-      alt="Blockly icon"
+      alt="An if do block where the if input is true and the do input sets border radius to 24"
       src="/images/codelabs/card_thumbnails_css.png"
       srcDark="/images/codelabs/card_thumbnails_css-dark.png"
     />
@@ -75,7 +75,7 @@ Blockly Codelabs provide a guided, tutorial, hands-on coding experience of Block
     title="Build a custom generator"
     href="/codelabs/custom-generator/codelab-overview">
     <Image
-      alt="Blockly icon"
+      alt="A create list block with four inputs: the number one, the string 'two', boolean false, and boolean true"
       src="/images/codelabs/card_thumbnails_custom-generator.png"
       srcDark="/images/codelabs/card_thumbnails_custom-generator-dark.png"
     />
@@ -85,7 +85,7 @@ Blockly Codelabs provide a guided, tutorial, hands-on coding experience of Block
     title="Build custom renderers"
     href="/codelabs/custom-renderer/codelab-overview">
     <Image
-      alt="Blockly icon"
+      alt="A repeat 10 times loop block"
       src="/images/codelabs/card_thumbnails_custom-renderers.png"
       srcDark="/images/codelabs/card_thumbnails_custom-renderers-dark.png"
     />