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_test.go

@@ -14,6 +14,7 @@
 package page
 
 import (
+	"context"
 	"fmt"
 	"testing"
 	"time"
@@ -104,6 +105,12 @@ func TestSortByN(t *testing.T) {
 	d4 := d1.Add(-20 * time.Hour)
 
 	p := createSortTestPages(4)
+	ctx := context.Background()
+
+	byLen := func(p Pages) Pages {
+		return p.ByLength(ctx)
+
+	}
 
 	for i, this := range []struct {
 		sortFunc   func(p Pages) Pages
@@ -116,7 +123,7 @@ func TestSortByN(t *testing.T) {
 		{(Pages).ByPublishDate, func(p Pages) bool { return p[0].PublishDate() == d4 }},
 		{(Pages).ByExpiryDate, func(p Pages) bool { return p[0].ExpiryDate() == d4 }},
 		{(Pages).ByLastmod, func(p Pages) bool { return p[1].Lastmod() == d3 }},
-		{(Pages).ByLength, func(p Pages) bool { return p[0].(resource.LengthProvider).Len() == len(p[0].(*testPage).content) }},
+		{byLen, func(p Pages) bool { return p[0].(resource.LengthProvider).Len(ctx) == len(p[0].(*testPage).content) }},
 	} {
 		setSortVals([4]time.Time{d1, d2, d3, d4}, [4]string{"b", "ab", "cde", "fg"}, [4]int{0, 3, 2, 1}, p)