docs: improve text readability on examples page (#5182)

## Summary
- Add darker overlay for image generation and document processing cards
to improve text readability on light-colored cover images
- Maintain same gradient positioning as base overlay while increasing
opacity values

## Before/After Screenshots
<img width="1070" height="945" alt="image"
src="https://github.com/user-attachments/assets/7ef48940-fa07-4c14-a4a9-092d1e9bb274"
/>

<img width="1066" height="947" alt="image"
src="https://github.com/user-attachments/assets/643bfbba-2b78-48ae-94ae-ae2039820cf8"
/>

## Test plan
- [x] Verify text is readable on all example cards
- [x] Check overlay doesn't obscure image details unnecessarily
- [x] Test responsive behavior on mobile

## Internal
Closes
https://linear.app/eventual/issue/EVE-875/darken-the-background-overlay-for-the-text-for-examples

Author: ykdojo

docs/examples/index.md

@@ -5,7 +5,7 @@
     <a href="./document-processing" class="example-image-link">
       <div class="example-image">
         <img src="../img/document-processing-cover.jpg" alt="Document Processing">
-        <div class="example-overlay">
+        <div class="example-overlay example-overlay-dark">
           <h3>Document Processing</h3>
           <p>Load PDFs from S3, extract text, run layout analysis, and compute embeddings</p>
         </div>
@@ -62,7 +62,7 @@
         <div class="example-placeholder" style="display: none; background: linear-gradient(135deg, #4facfe 0%, #00f2fe 100%);">
           <span>🎨</span>
         </div>
-        <div class="example-overlay">
+        <div class="example-overlay example-overlay-dark">
           <h3>Generate Images with Stable Diffusion</h3>
           <p>Using text prompts with deep learning models</p>
         </div>
@@ -219,6 +219,10 @@
   line-height: 1.4;
 }
 
+.example-overlay-dark {
+  background: linear-gradient(to top, rgba(0, 0, 0, 0.9) 12%, rgba(0, 0, 0, 0.6) 35%, rgba(0, 0, 0, 0) 65%);
+}
+
 @media (max-width: 768px) {
   .examples-grid {
     grid-template-columns: 1fr;