fix(what-is): make CTA heading and text readable on light background (#18873)

The "Get started today" CTA card on /what-is/* pages uses a light
violet background (bg-violet-50) but the heading and paragraph were
set to text-white, making them invisible. Switch to text-gray-900 for
the heading and text-gray-700 for the paragraph so they're legible.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: alexleventer

layouts/what-is/single.html

@@ -43,9 +43,9 @@ <h4>{{ .title }}</h4>
 <section id="get-started" class="container px-6 lg:px-0 mx-auto my-16">
     <div class="w-full bg-violet-50 card p-6 lg:p-16 lg:pt-24 text-center">
         <div class="max-w-xl mx-auto">
-                <h2 class="text-white hidden lg:block px-0 lg:px-16">Get started today</h2>
-                <h4 class="text-white mt-0 lg:hidden">Get started today</h4>
-                <p class="text-white">Pulumi is open source and free to get started. Deploy your first stack today.</p>
+                <h2 class="text-gray-900 hidden lg:block px-0 lg:px-16">Get started today</h2>
+                <h4 class="text-gray-900 mt-0 lg:hidden">Get started today</h4>
+                <p class="text-gray-700">Pulumi is open source and free to get started. Deploy your first stack today.</p>
                 <div class="mt-16">
                     <a class="btn" href="{{ site.Params.cta.primary.href }}" data-role="cta-get-started">{{ site.Params.cta.primary.label }}</a>
                 </div>