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

related/integration_test.go

@@ -21,6 +21,123 @@ import (
 	"github.com/gohugoio/hugo/hugolib"
 )
 
+func TestRelatedFragments(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+baseURL = "http://example.com/"
+disableKinds = ["taxonomy", "term", "RSS", "sitemap", "robotsTXT"]
+[related]
+  includeNewer = false
+  threshold = 80
+  toLower = false
+[[related.indices]]
+  name = 'pagerefs'
+  type = 'fragments'
+  applyFilter = true
+  weight = 90
+[[related.indices]]
+  name = 'keywords'
+  weight = 80	
+-- content/p1.md --
+---
+title: p1
+pagerefs: ['ref1']
+---
+{{< see-also >}}
+
+## P1 title
+
+-- content/p2.md --
+---
+title: p2
+---
+
+## P2 title 1
+
+## P2 title 2
+
+## First title {#ref1}
+{{< see-also "ref1" >}}
+-- content/p3.md --
+---
+title: p3
+keywords: ['foo']
+---
+
+## P3 title 1
+
+## P3 title 2
+
+## Common p3, p4, p5
+-- content/p4.md --
+---
+title: p4
+---
+
+## Common p3, p4, p5
+
+## P4 title 1
+
+-- content/p5.md --
+---
+title: p5
+keywords: ['foo']
+---
+
+## P5 title 1
+
+## Common p3, p4, p5
+
+-- layouts/shortcodes/see-also.html --
+{{ $p1 := site.GetPage "p1" }}
+{{ $p2 := site.GetPage "p2" }}
+{{ $p3 := site.GetPage "p3" }}
+P1 Fragments: {{ $p1.Fragments.Identifiers }}
+P2 Fragments: {{ $p2.Fragments.Identifiers }}
+Contains ref1: {{ $p2.Fragments.Identifiers.Contains "ref1" }}
+Count ref1: {{ $p2.Fragments.Identifiers.Count "ref1" }}
+{{ $opts := dict "document" .Page "fragments" $.Params }}
+{{ $related1 := site.RegularPages.Related $opts }}
+{{ $related2 := site.RegularPages.Related $p3 }}
+Len Related 1: {{ len $related1 }}
+Len Related 2: {{ len $related2 }}
+Related 1: {{ template "list-related" $related1 }}
+Related 2: {{ template "list-related" $related2 }}
+
+{{ define "list-related" }}{{ range $i, $e := . }} {{ $i }}: {{ .Title }}: {{ with .HeadingsFiltered}}{{ range $i, $e := .}}h{{ $i }}: {{ .Title }}|{{ .ID }}|{{ end }}{{ end }}::END{{ end }}{{ end }}
+
+-- layouts/_default/single.html --
+Content: {{ .Content }}
+
+	
+`
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+		}).Build()
+
+	expect := `
+P1 Fragments: [p1-title]	
+P2 Fragments: [p2-title-1 p2-title-2 ref1]
+Len Related 1: 1
+Related 2: 2
+`
+
+	for _, p := range []string{"p1", "p2"} {
+		b.AssertFileContent("public/"+p+"/index.html", expect)
+	}
+
+	b.AssertFileContent("public/p1/index.html",
+		"Related 1:  0: p2: h0: First title|ref1|::END",
+		"Related 2:  0: p5: h0: Common p3, p4, p5|common-p3-p4-p5|::END 1: p4: h0: Common p3, p4, p5|common-p3-p4-p5|::END",
+	)
+
+}
+
 func BenchmarkRelatedSite(b *testing.B) {
 	files := `
 -- config.toml --
@@ -33,6 +150,10 @@ disableKinds = ["taxonomy", "term", "RSS", "sitemap", "robotsTXT"]
 [[related.indices]]
   name = 'keywords'
   weight = 70	
+[[related.indices]]
+  name = 'pagerefs'
+  type = 'fragments'
+  weight = 30	
 -- layouts/_default/single.html --
 {{ range site.RegularPages }}
 {{ $tmp := .WordCount }}