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/shortcode_page.go

@@ -14,13 +14,48 @@
 package hugolib
 
 import (
+	"context"
 	"html/template"
 
 	"github.com/gohugoio/hugo/resources/page"
 )
 
+// A placeholder for the TableOfContents markup. This is what we pass to the Goldmark etc. renderers.
 var tocShortcodePlaceholder = createShortcodePlaceholder("TOC", 0)
 
+// shortcodeRenderer is typically used to delay rendering of inner shortcodes
+// marked with placeholders in the content.
+type shortcodeRenderer interface {
+	renderShortcode(context.Context) ([]byte, bool, error)
+	renderShortcodeString(context.Context) (string, bool, error)
+}
+
+type shortcodeRenderFunc func(context.Context) ([]byte, bool, error)
+
+func (f shortcodeRenderFunc) renderShortcode(ctx context.Context) ([]byte, bool, error) {
+	return f(ctx)
+}
+
+func (f shortcodeRenderFunc) renderShortcodeString(ctx context.Context) (string, bool, error) {
+	b, has, err := f(ctx)
+	return string(b), has, err
+}
+
+type prerenderedShortcode struct {
+	s           string
+	hasVariants bool
+}
+
+func (p prerenderedShortcode) renderShortcode(context.Context) ([]byte, bool, error) {
+	return []byte(p.s), p.hasVariants, nil
+}
+
+func (p prerenderedShortcode) renderShortcodeString(context.Context) (string, bool, error) {
+	return p.s, p.hasVariants, nil
+}
+
+var zeroShortcode = prerenderedShortcode{}
+
 // This is sent to the shortcodes. They cannot access the content
 // they're a part of. It would cause an infinite regress.
 //
@@ -50,7 +85,11 @@ func (p *pageForShortcode) page() page.Page {
 	return p.PageWithoutContent.(page.Page)
 }
 
-func (p *pageForShortcode) TableOfContents() template.HTML {
+func (p *pageForShortcode) String() string {
+	return p.p.String()
+}
+
+func (p *pageForShortcode) TableOfContents(context.Context) template.HTML {
 	p.p.enablePlaceholders()
 	return p.toc
 }