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/page.go

@@ -16,10 +16,12 @@
 package page
 
 import (
+	"context"
 	"html/template"
 
 	"github.com/gohugoio/hugo/identity"
 	"github.com/gohugoio/hugo/markup/converter"
+	"github.com/gohugoio/hugo/markup/tableofcontents"
 
 	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/tpl"
@@ -76,40 +78,40 @@ type ChildCareProvider interface {
 
 // ContentProvider provides the content related values for a Page.
 type ContentProvider interface {
-	Content() (any, error)
+	Content(context.Context) (any, error)
 
 	// Plain returns the Page Content stripped of HTML markup.
-	Plain() string
+	Plain(context.Context) string
 
 	// PlainWords returns a string slice from splitting Plain using https://pkg.go.dev/strings#Fields.
-	PlainWords() []string
+	PlainWords(context.Context) []string
 
 	// Summary returns a generated summary of the content.
 	// The breakpoint can be set manually by inserting a summary separator in the source file.
-	Summary() template.HTML
+	Summary(context.Context) template.HTML
 
 	// Truncated returns whether the Summary  is truncated or not.
-	Truncated() bool
+	Truncated(context.Context) bool
 
 	// FuzzyWordCount returns the approximate number of words in the content.
-	FuzzyWordCount() int
+	FuzzyWordCount(context.Context) int
 
 	// WordCount returns the number of words in the content.
-	WordCount() int
+	WordCount(context.Context) int
 
 	// ReadingTime returns the reading time based on the length of plain text.
-	ReadingTime() int
+	ReadingTime(context.Context) int
 
 	// Len returns the length of the content.
 	// This is for internal use only.
-	Len() int
+	Len(context.Context) int
 }
 
 // ContentRenderer provides the content rendering methods for some content.
 type ContentRenderer interface {
 	// RenderContent renders the given content.
 	// For internal use only.
-	RenderContent(content []byte, renderTOC bool) (converter.Result, error)
+	RenderContent(ctx context.Context, content []byte, renderTOC bool) (converter.Result, error)
 }
 
 // FileProvider provides the source file.
@@ -167,6 +169,11 @@ type Page interface {
 	PageWithoutContent
 }
 
+type PageFragment interface {
+	resource.ResourceLinksProvider
+	resource.ResourceMetaProvider
+}
+
 // PageMetaProvider provides page metadata, typically provided via front matter.
 type PageMetaProvider interface {
 	// The 4 page dates
@@ -252,7 +259,7 @@ type PageMetaProvider interface {
 // PageRenderProvider provides a way for a Page to render content.
 type PageRenderProvider interface {
 	// Render renders the given layout with this Page as context.
-	Render(layout ...string) (template.HTML, error)
+	Render(ctx context.Context, layout ...string) (template.HTML, error)
 	// RenderString renders the first value in args with tPaginatorhe content renderer defined
 	// for this Page.
 	// It takes an optional map as a second argument:
@@ -260,7 +267,7 @@ type PageRenderProvider interface {
 	// display (“inline”):
 	// - inline or block. If inline (default), surrounding <p></p> on short snippets will be trimmed.
 	// markup (defaults to the Page’s markup)
-	RenderString(args ...any) (template.HTML, error)
+	RenderString(ctx context.Context, args ...any) (template.HTML, error)
 }
 
 // PageWithoutContent is the Page without any of the content methods.
@@ -323,6 +330,14 @@ type PageWithoutContent interface {
 	// Used in change/dependency tracking.
 	identity.Provider
 
+	// Fragments returns the fragments for this page.
+	Fragments(context.Context) *tableofcontents.Fragments
+
+	// Headings returns the headings for this page when a filter is set.
+	// This is currently only triggered with the Related content feature
+	// and the "fragments" type of index.
+	HeadingsFiltered(context.Context) tableofcontents.Headings
+
 	DeprecatedWarningPageMethods
 }
 
@@ -387,7 +402,7 @@ type SitesProvider interface {
 // TableOfContentsProvider provides the table of contents for a Page.
 type TableOfContentsProvider interface {
 	// TableOfContents returns the table of contents for the page rendered as HTML.
-	TableOfContents() template.HTML
+	TableOfContents(context.Context) template.HTML
 }
 
 // TranslationsProvider provides access to any translations.