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

1 files changed, 10 insertions, 4 deletions

diff --git a/hugolib/page__content.go b/hugolib/page__content.go
index a721d1fce..89c38bd84 100644
--- a/hugolib/page__content.go
+++ b/hugolib/page__content.go
@@ -14,6 +14,7 @@
 package hugolib
 
 import (
+	"context"
 	"fmt"
 
 	"github.com/gohugoio/hugo/output"
@@ -37,9 +38,9 @@ type pageContent struct {
 }
 
 // returns the content to be processed by Goldmark or similar.
-func (p pageContent) contentToRender(parsed pageparser.Result, pm *pageContentMap, renderedShortcodes map[string]string) []byte {
+func (p pageContent) contentToRender(ctx context.Context, parsed pageparser.Result, pm *pageContentMap, renderedShortcodes map[string]shortcodeRenderer) ([]byte, bool, error) {
 	source := parsed.Input()
-
+	var hasVariants bool
 	c := make([]byte, 0, len(source)+(len(source)/10))
 
 	for _, it := range pm.items {
@@ -57,7 +58,12 @@ func (p pageContent) contentToRender(parsed pageparser.Result, pm *pageContentMa
 					panic(fmt.Sprintf("rendered shortcode %q not found", v.placeholder))
 				}
 
-				c = append(c, []byte(renderedShortcode)...)
+				b, more, err := renderedShortcode.renderShortcode(ctx)
+				if err != nil {
+					return nil, false, fmt.Errorf("failed to render shortcode: %w", err)
+				}
+				hasVariants = hasVariants || more
+				c = append(c, []byte(b)...)
 
 			} else {
 				// Insert the placeholder so we can insert the content after
@@ -69,7 +75,7 @@ func (p pageContent) contentToRender(parsed pageparser.Result, pm *pageContentMa
 		}
 	}
 
-	return c
+	return c, hasVariants, nil
 }
 
 func (p pageContent) selfLayoutForOutput(f output.Format) string {