Fix IDE0370: remove unnecessary null-forgiving operators in SiteNavigationV2

The pattern `{ Page: var page }` combined with the prior `{ Page: null }` arm
already guarantees `page` is non-null when this arm matches.

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

Author: theletterf

src/Elastic.Documentation.Navigation/V2/SiteNavigationV2.cs

@@ -60,7 +60,7 @@ string sitePrefix
 			GroupNavV2Item group => CreateGroup(group, nodes, parent, depth, sitePrefix),
 			TocNavV2Item toc => nodes.TryGetValue(toc.Source, out var node) ? node : null,
 			PageNavV2Item { Page: null, Title: var title } => new PlaceholderNavigationLeaf(title ?? "Untitled", parent),
-			PageNavV2Item { Page: var page, Title: var title } => new PageCrossLinkLeaf(page!, title ?? page!.ToString(), sitePrefix, parent),
+			PageNavV2Item { Page: var page, Title: var title } => new PageCrossLinkLeaf(page, title ?? page.ToString(), sitePrefix, parent),
 			_ => null
 		};