Add DiagnosticNote to make warnings more actionable

Enhance the "symbols with @TechnologyRoot pages" warning by adding
DiagnosticNote entries that point to the symbol graph files causing
the multiple roots issue. This helps developers identify exactly
which symbol graph files are part of the problem.

Also update the test to verify the diagnostic notes are present.

Based on PR review feedback.

Author: agisilaos

Sources/SwiftDocC/Infrastructure/DocumentationContext.swift

@@ -2433,6 +2433,17 @@ public class DocumentationContext {
         // This is an unsupported setup that creates multiple roots in the documentation hierarchy
         if !rootModules.isEmpty && !rootPageArticles.isEmpty {
             let problems = rootPageArticles.map { rootPageArticle -> Problem in
+                // Create notes pointing to symbol graph files that are causing the multiple roots issue
+                let symbolGraphNotes: [DiagnosticNote] = bundle.symbolGraphURLs.map { symbolGraphURL in
+                    let fileName = symbolGraphURL.lastPathComponent
+                    let zeroRange = SourceLocation(line: 1, column: 1, source: nil)..<SourceLocation(line: 1, column: 1, source: nil)
+                    return DiagnosticNote(
+                        source: symbolGraphURL,
+                        range: zeroRange,
+                        message: "Symbol graph file '\(fileName)' creates a module root"
+                    )
+                }
+                
                 let diagnostic = Diagnostic(
                     source: rootPageArticle.source,
                     severity: .warning,
@@ -2442,7 +2453,8 @@ public class DocumentationContext {
                     explanation: """
                     When documentation contains symbols (from symbol graph files), @TechnologyRoot directives create an unsupported setup with multiple roots in the documentation hierarchy.
                     Remove the @TechnologyRoot directive so that this page is treated as an article under the module.
-                    """
+                    """,
+                    notes: symbolGraphNotes
                 )
 
                 guard let range = rootPageArticle.value.metadata?.technologyRoot?.originalMarkup.range else {

Tests/SwiftDocCTests/Infrastructure/DocumentationContext/DocumentationContext+RootPageTests.swift

@@ -257,6 +257,12 @@ class DocumentationContext_RootPageTests: XCTestCase {
             let solution = try XCTUnwrap(problem.possibleSolutions.first)
             XCTAssertEqual(solution.summary, "Remove the 'TechnologyRoot' directive")
             XCTAssertEqual(solution.replacements.count, 1)
+            
+            // Verify that diagnostic notes point to the symbol graph file
+            XCTAssertEqual(problem.diagnostic.notes.count, 1, "Should have a note pointing to the symbol graph file")
+            let note = try XCTUnwrap(problem.diagnostic.notes.first)
+            XCTAssertTrue(note.source.lastPathComponent.hasSuffix(".symbols.json"), "Note should point to a symbol graph file")
+            XCTAssertTrue(note.message.contains("Symbol graph file"), "Note should mention symbol graph file")
         }
     }