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

hugolib/page.go

@@ -15,6 +15,7 @@ package hugolib
 
 import (
 	"bytes"
+	"context"
 	"fmt"
 	"path"
 	"path/filepath"
@@ -24,8 +25,10 @@ import (
 	"go.uber.org/atomic"
 
 	"github.com/gohugoio/hugo/identity"
+	"github.com/gohugoio/hugo/related"
 
 	"github.com/gohugoio/hugo/markup/converter"
+	"github.com/gohugoio/hugo/markup/tableofcontents"
 
 	"github.com/gohugoio/hugo/tpl"
 
@@ -148,6 +151,43 @@ func (p *pageState) GetIdentity() identity.Identity {
 	return identity.NewPathIdentity(files.ComponentFolderContent, filepath.FromSlash(p.Pathc()))
 }
 
+func (p *pageState) Fragments(ctx context.Context) *tableofcontents.Fragments {
+	p.s.initInit(ctx, p.cp.initToC, p)
+	if p.pageOutput.cp.tableOfContents == nil {
+		return tableofcontents.Empty
+	}
+	return p.pageOutput.cp.tableOfContents
+}
+
+func (p *pageState) HeadingsFiltered(context.Context) tableofcontents.Headings {
+	return nil
+}
+
+type pageHeadingsFiltered struct {
+	*pageState
+	headings tableofcontents.Headings
+}
+
+func (p *pageHeadingsFiltered) HeadingsFiltered(context.Context) tableofcontents.Headings {
+	return p.headings
+}
+
+func (p *pageHeadingsFiltered) page() page.Page {
+	return p.pageState
+}
+
+// For internal use by the related content feature.
+func (p *pageState) ApplyFilterToHeadings(ctx context.Context, fn func(*tableofcontents.Heading) bool) related.Document {
+	if p.pageOutput.cp.tableOfContents == nil {
+		return p
+	}
+	headings := p.pageOutput.cp.tableOfContents.Headings.FilterBy(fn)
+	return &pageHeadingsFiltered{
+		pageState: p,
+		headings:  headings,
+	}
+}
+
 func (p *pageState) GitInfo() source.GitInfo {
 	return p.gitInfo
 }
@@ -351,7 +391,7 @@ func (p *pageState) String() string {
 // IsTranslated returns whether this content file is translated to
 // other language(s).
 func (p *pageState) IsTranslated() bool {
-	p.s.h.init.translations.Do()
+	p.s.h.init.translations.Do(context.Background())
 	return len(p.translations) > 0
 }
 
@@ -375,13 +415,13 @@ func (p *pageState) TranslationKey() string {
 
 // AllTranslations returns all translations, including the current Page.
 func (p *pageState) AllTranslations() page.Pages {
-	p.s.h.init.translations.Do()
+	p.s.h.init.translations.Do(context.Background())
 	return p.allTranslations
 }
 
 // Translations returns the translations excluding the current Page.
 func (p *pageState) Translations() page.Pages {
-	p.s.h.init.translations.Do()
+	p.s.h.init.translations.Do(context.Background())
 	return p.translations
 }
 
@@ -461,7 +501,7 @@ func (p *pageState) initOutputFormat(isRenderingSite bool, idx int) error {
 
 // Must be run after the site section tree etc. is built and ready.
 func (p *pageState) initPage() error {
-	if _, err := p.init.Do(); err != nil {
+	if _, err := p.init.Do(context.Background()); err != nil {
 		return err
 	}
 	return nil