Find every HeadingBlock in a document (#349)

* Find every HeadingBlock in a document

* Add test

src/Elastic.Markdown/IO/MarkdownFile.cs

@@ -162,8 +162,8 @@ private void ReadDocumentInstructions(MarkdownDocument document)
 		}
 
 		var contents = document
-			.Where(block => block is HeadingBlock { Level: >= 2 })
-			.Cast<HeadingBlock>()
+			.Descendants<HeadingBlock>()
+			.Where(block => block is { Level: >= 2 })
 			.Select(h => (h.GetData("header") as string, h.GetData("anchor") as string))
 			.Select(h => new PageTocItem
 			{

tests/Elastic.Markdown.Tests/Inline/AnchorLinkTests.cs

@@ -34,6 +34,13 @@ protected override void AddToFileSystem(MockFileSystem fileSystem)
 
 ## New Requirements [#new-reqs]
 
+
+:::{dropdown} Nested heading
+
+##### Heading inside dropdown [#heading-inside-dropdown]
+
+:::
+
 These are new requirements
 """;
 		fileSystem.AddFile(@"docs/testing/req.md", inclusion);
@@ -147,3 +154,21 @@ public void GeneratesHtml() =>
 	public void HasError() => Collector.Diagnostics.Should().HaveCount(1)
 		.And.Contain(d => d.Message.Contains("`sub-requirements2` does not exist"));
 }
+
+
+public class NestedHeadingTest(ITestOutputHelper output) : AnchorLinkTestBase(output,
+	"""
+	[Heading inside dropdown](testing/req.md#heading-inside-dropdown)
+	"""
+)
+{
+	[Fact]
+	public void GeneratesHtml() =>
+		// language=html
+		Html.Should().Contain(
+			"""<a href="testing/req.html#heading-inside-dropdown">Heading inside dropdown</a>"""
+		);
+
+	[Fact]
+	public void HasError() => Collector.Diagnostics.Should().HaveCount(0);
+}