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_related_test.go

@@ -14,6 +14,7 @@
 package page
 
 import (
+	"context"
 	"testing"
 	"time"
 
@@ -51,26 +52,42 @@ func TestRelated(t *testing.T) {
 		},
 	}
 
-	result, err := pages.RelatedTo(types.NewKeyValuesStrings("keywords", "hugo", "rocks"))
+	ctx := context.Background()
+	opts := map[string]any{
+		"namedSlices": types.NewKeyValuesStrings("keywords", "hugo", "rocks"),
+	}
+	result, err := pages.Related(ctx, opts)
 
 	c.Assert(err, qt.IsNil)
 	c.Assert(len(result), qt.Equals, 2)
 	c.Assert(result[0].Title(), qt.Equals, "Page 2")
 	c.Assert(result[1].Title(), qt.Equals, "Page 1")
 
-	result, err = pages.Related(pages[0])
+	result, err = pages.Related(ctx, pages[0])
 	c.Assert(err, qt.IsNil)
 	c.Assert(len(result), qt.Equals, 2)
 	c.Assert(result[0].Title(), qt.Equals, "Page 2")
 	c.Assert(result[1].Title(), qt.Equals, "Page 3")
 
-	result, err = pages.RelatedIndices(pages[0], "keywords")
+	opts = map[string]any{
+		"document": pages[0],
+		"indices":  []string{"keywords"},
+	}
+	result, err = pages.Related(ctx, opts)
 	c.Assert(err, qt.IsNil)
 	c.Assert(len(result), qt.Equals, 2)
 	c.Assert(result[0].Title(), qt.Equals, "Page 2")
 	c.Assert(result[1].Title(), qt.Equals, "Page 3")
 
-	result, err = pages.RelatedTo(types.NewKeyValuesStrings("keywords", "bep", "rocks"))
+	opts = map[string]any{
+		"namedSlices": []types.KeyValues{
+			{
+				Key:    "keywords",
+				Values: []any{"bep", "rocks"},
+			},
+		},
+	}
+	result, err = pages.Related(context.Background(), opts)
 	c.Assert(err, qt.IsNil)
 	c.Assert(len(result), qt.Equals, 2)
 	c.Assert(result[0].Title(), qt.Equals, "Page 2")