Update Markdown preview examples to include tabindex (#3330)

Author: khiga8

app/components/primer/beta/markdown.rb

@@ -3,6 +3,7 @@
 module Primer
   module Beta
     # Use `Markdown` to wrap markdown content.
+    # @accessibility This component is purely presentational. Consumers should handle accessibility expectations, such as ensuring that an overflowing, scrollable code block or table is keyboard accessible.
     class Markdown < Primer::Component
       status :beta
 

previews/primer/beta/markdown_preview.rb

@@ -181,7 +181,7 @@ class MarkdownPreview < ViewComponent::Preview
 
       <p>If a table is too wide, it should condense down and/or scroll horizontally.</p>
 
-      <table>
+      <table tabindex="0">
         <thead>
           <tr>
             <th>Artist</th>
@@ -237,13 +237,13 @@ class MarkdownPreview < ViewComponent::Preview
 
       <pre><code>var foo = \"bar\";</code></pre>
 
-      <pre><code>Long, single-line code blocks should not wrap. They should horizontally scroll if they are too long. This line should be long enough to demonstrate this.</code></pre>
+      <pre tabindex="0"><code>Long, single-line code blocks should not wrap. They should horizontally scroll if they are too long. They should also have a tabindex=0 to ensure keyboard accessibility. This line should be long enough to demonstrate this.</code></pre>
 
-      <pre><code>var foo = \"The same thing is true for code with syntax highlighting. A single line of code should horizontally scroll if it is really long.\";</code></pre>
+      <pre tabindex="0"><code>var foo = \"The same thing is true for code with syntax highlighting. A single line of code should horizontally scroll if it is really long.\";</code></pre>
 
       <p>Inline code inside table cells should still be distinguishable.</p>
 
-      <table>
+      <table tabindex="0">
         <thead>
           <tr>
             <th>Language</th>

static/info_arch.json

@@ -14520,7 +14520,7 @@
   {
     "fully_qualified_name": "Primer::Beta::Markdown",
     "description": "Use `Markdown` to wrap markdown content.",
-    "accessibility_docs": null,
+    "accessibility_docs": "This component is purely presentational. Consumers should handle accessibility expectations, such as ensuring that an overflowing, scrollable code block or table is keyboard accessible.",
     "is_form_component": false,
     "is_published": true,
     "requires_js": false,