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/page/pages_sort.go

@@ -14,6 +14,7 @@
 package page
 
 import (
+	"context"
 	"sort"
 
 	"github.com/gohugoio/hugo/common/collections"
@@ -299,7 +300,7 @@ func (p Pages) ByLastmod() Pages {
 // Adjacent invocations on the same receiver will return a cached result.
 //
 // This may safely be executed  in parallel.
-func (p Pages) ByLength() Pages {
+func (p Pages) ByLength(ctx context.Context) Pages {
 	const key = "pageSort.ByLength"
 
 	length := func(p1, p2 Page) bool {
@@ -314,7 +315,7 @@ func (p Pages) ByLength() Pages {
 			return false
 		}
 
-		return p1l.Len() < p2l.Len()
+		return p1l.Len(ctx) < p2l.Len(ctx)
 	}
 
 	pages, _ := spc.get(key, pageBy(length).Sort, p)