Add page fragments support to Related

The main topic of this commit is that you can now index fragments (content heading identifiers) when calling `.Related`.

You can do this by:

* Configure one or more indices with type `fragments`
* The name of those index configurations maps to an (optional) front matter slice with fragment references. This allows you to link
page<->fragment and page<->page.
* This also will index all the fragments (heading identifiers) of the pages.

It's also possible to use type `fragments` indices in shortcode, e.g.:

```
{{ $related := site.RegularPages.Related .Page }}
```

But, and this is important, you need to include the shortcode using the `{{<` delimiter. Not doing so will create infinite loops and timeouts.

This commit also:

* Adds two new methods to Page: Fragments (can also be used to build ToC) and HeadingsFiltered (this is only used in Related Content with
index type `fragments` and `enableFilter` set to true.
* Consolidates all `.Related*` methods into one, which takes either a `Page` or an options map as its only argument.
* Add `context.Context` to all of the content related Page API. Turns out it wasn't strictly needed for this particular feature, but it will
soon become usefil, e.g. in #9339.

Closes #10711
Updates #9339
Updates #10725

markup/goldmark/toc.go

@@ -41,8 +41,8 @@ func (t *tocTransformer) Transform(n *ast.Document, reader text.Reader, pc parse
 	}
 
 	var (
-		toc         tableofcontents.Root
-		tocHeading  tableofcontents.Heading
+		toc         tableofcontents.Builder
+		tocHeading  = &tableofcontents.Heading{}
 		level       int
 		row         = -1
 		inHeading   bool
@@ -53,10 +53,10 @@ func (t *tocTransformer) Transform(n *ast.Document, reader text.Reader, pc parse
 		s := ast.WalkStatus(ast.WalkContinue)
 		if n.Kind() == ast.KindHeading {
 			if inHeading && !entering {
-				tocHeading.Text = headingText.String()
+				tocHeading.Title = headingText.String()
 				headingText.Reset()
 				toc.AddAt(tocHeading, row, level-1)
-				tocHeading = tableofcontents.Heading{}
+				tocHeading = &tableofcontents.Heading{}
 				inHeading = false
 				return s, nil
 			}
@@ -106,7 +106,7 @@ func (t *tocTransformer) Transform(n *ast.Document, reader text.Reader, pc parse
 		return s, nil
 	})
 
-	pc.Set(tocResultKey, toc)
+	pc.Set(tocResultKey, toc.Build())
 }
 
 type tocExtension struct {