Move Deep Search into main nav section (#1243)

Discussion:
https://sourcegraph.slack.com/archives/C08SNU0M815/p1751313174458049

Deep Search is now in the main nav section and looks like this:
<img width="286" alt="image"
src="https://github.com/user-attachments/assets/b3e5a3d2-ab2c-4dd8-8fff-1b53997e2c21"
/>

Potential followups: should we break up the page into more detailed
docs?

Author: janhartman

docs/code-search/types/deep-search.mdx renamed to docs/deep-search/index.mdx

@@ -252,10 +252,6 @@ export const navigation: NavigationItem[] = [
 							{
 								title: 'Structural Search',
 								href: '/code-search/types/structural'
-							},
-							{
-								title: 'Deep Search',
-								href: '/code-search/types/deep-search'
 							}
 						]
 					},
@@ -264,6 +260,16 @@ export const navigation: NavigationItem[] = [
 						href: '/code-search/faq'
 					}
 				]
+			},
+			{
+				title: 'Deep Search',
+				href: '/deep-search',
+				sections: [
+					{
+						title: 'Deep Search',
+						href: '/deep-search'
+					}
+				]
 			}
 		]
 	},