docs: Add descriptive intro text to index pages

Adds brief explanatory paragraphs to Jobs, Materials, and Stock index
pages to help users understand each section's purpose.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CutList.Web/Components/Pages/Jobs/Index.razor

@@ -18,6 +18,11 @@
     </div>
 </div>
 
+<p class="text-muted mb-4">
+    Jobs organize the parts you need to cut for a project. Add parts with their required lengths and quantities,
+    assign stock materials, then run the optimizer to generate an efficient cut list that minimizes waste.
+</p>
+
 @if (loading)
 {
     <p><em>Loading...</em></p>