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/Materials/Index.razor

@@ -9,6 +9,11 @@
     <a href="materials/new" class="btn btn-primary">Add Material</a>
 </div>
 
+<p class="text-muted mb-4">
+    Manage your material catalog here. Materials define the types of stock you work with — shape, size, type, and
+    grade. Once added, materials can be assigned to jobs and used to generate optimized cut lists.
+</p>
+
 @if (loading)
 {
     <p><em>Loading...</em></p>