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_lazy_contentprovider.go

@@ -14,6 +14,7 @@
 package page
 
 import (
+	"context"
 	"html/template"
 
 	"github.com/gohugoio/hugo/lazy"
@@ -57,7 +58,7 @@ func NewLazyContentProvider(f func() (OutputFormatContentProvider, error)) *Lazy
 		init: lazy.New(),
 		cp:   NopCPageContentRenderer,
 	}
-	lcp.init.Add(func() (any, error) {
+	lcp.init.Add(func(context.Context) (any, error) {
 		cp, err := f()
 		if err != nil {
 			return nil, err
@@ -72,67 +73,67 @@ func (lcp *LazyContentProvider) Reset() {
 	lcp.init.Reset()
 }
 
-func (lcp *LazyContentProvider) Content() (any, error) {
-	lcp.init.Do()
-	return lcp.cp.Content()
+func (lcp *LazyContentProvider) Content(ctx context.Context) (any, error) {
+	lcp.init.Do(ctx)
+	return lcp.cp.Content(ctx)
 }
 
-func (lcp *LazyContentProvider) Plain() string {
-	lcp.init.Do()
-	return lcp.cp.Plain()
+func (lcp *LazyContentProvider) Plain(ctx context.Context) string {
+	lcp.init.Do(ctx)
+	return lcp.cp.Plain(ctx)
 }
 
-func (lcp *LazyContentProvider) PlainWords() []string {
-	lcp.init.Do()
-	return lcp.cp.PlainWords()
+func (lcp *LazyContentProvider) PlainWords(ctx context.Context) []string {
+	lcp.init.Do(ctx)
+	return lcp.cp.PlainWords(ctx)
 }
 
-func (lcp *LazyContentProvider) Summary() template.HTML {
-	lcp.init.Do()
-	return lcp.cp.Summary()
+func (lcp *LazyContentProvider) Summary(ctx context.Context) template.HTML {
+	lcp.init.Do(ctx)
+	return lcp.cp.Summary(ctx)
 }
 
-func (lcp *LazyContentProvider) Truncated() bool {
-	lcp.init.Do()
-	return lcp.cp.Truncated()
+func (lcp *LazyContentProvider) Truncated(ctx context.Context) bool {
+	lcp.init.Do(ctx)
+	return lcp.cp.Truncated(ctx)
 }
 
-func (lcp *LazyContentProvider) FuzzyWordCount() int {
-	lcp.init.Do()
-	return lcp.cp.FuzzyWordCount()
+func (lcp *LazyContentProvider) FuzzyWordCount(ctx context.Context) int {
+	lcp.init.Do(ctx)
+	return lcp.cp.FuzzyWordCount(ctx)
 }
 
-func (lcp *LazyContentProvider) WordCount() int {
-	lcp.init.Do()
-	return lcp.cp.WordCount()
+func (lcp *LazyContentProvider) WordCount(ctx context.Context) int {
+	lcp.init.Do(ctx)
+	return lcp.cp.WordCount(ctx)
 }
 
-func (lcp *LazyContentProvider) ReadingTime() int {
-	lcp.init.Do()
-	return lcp.cp.ReadingTime()
+func (lcp *LazyContentProvider) ReadingTime(ctx context.Context) int {
+	lcp.init.Do(ctx)
+	return lcp.cp.ReadingTime(ctx)
 }
 
-func (lcp *LazyContentProvider) Len() int {
-	lcp.init.Do()
-	return lcp.cp.Len()
+func (lcp *LazyContentProvider) Len(ctx context.Context) int {
+	lcp.init.Do(ctx)
+	return lcp.cp.Len(ctx)
 }
 
-func (lcp *LazyContentProvider) Render(layout ...string) (template.HTML, error) {
-	lcp.init.Do()
-	return lcp.cp.Render(layout...)
+func (lcp *LazyContentProvider) Render(ctx context.Context, layout ...string) (template.HTML, error) {
+	lcp.init.Do(context.TODO())
+	return lcp.cp.Render(ctx, layout...)
 }
 
-func (lcp *LazyContentProvider) RenderString(args ...any) (template.HTML, error) {
-	lcp.init.Do()
-	return lcp.cp.RenderString(args...)
+func (lcp *LazyContentProvider) RenderString(ctx context.Context, args ...any) (template.HTML, error) {
+	lcp.init.Do(ctx)
+	return lcp.cp.RenderString(ctx, args...)
 }
 
-func (lcp *LazyContentProvider) TableOfContents() template.HTML {
-	lcp.init.Do()
-	return lcp.cp.TableOfContents()
+func (lcp *LazyContentProvider) TableOfContents(ctx context.Context) template.HTML {
+	lcp.init.Do(ctx)
+	return lcp.cp.TableOfContents(ctx)
 }
 
-func (lcp *LazyContentProvider) RenderContent(content []byte, renderTOC bool) (converter.Result, error) {
-	lcp.init.Do()
-	return lcp.cp.RenderContent(content, renderTOC)
+func (lcp *LazyContentProvider) RenderContent(ctx context.Context, content []byte, renderTOC bool) (converter.Result, error) {
+	lcp.init.Do(ctx)
+	return lcp.cp.RenderContent(ctx, content, renderTOC)
 }