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

resources/resource/resourcetypes.go

@@ -14,6 +14,8 @@
 package resource
 
 import (
+	"context"
+
 	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/langs"
 	"github.com/gohugoio/hugo/media"
@@ -162,7 +164,7 @@ type ContentProvider interface {
 	// * Page: template.HTML
 	// * JSON: String
 	// * Etc.
-	Content() (any, error)
+	Content(context.Context) (any, error)
 }
 
 // OpenReadSeekCloser allows setting some other way (than reading from a filesystem)
@@ -178,7 +180,7 @@ type ReadSeekCloserResource interface {
 // LengthProvider is a Resource that provides a length
 // (typically the length of the content).
 type LengthProvider interface {
-	Len() int
+	Len(context.Context) int
 }
 
 // LanguageProvider is a Resource in a language.