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, 4 insertions, 2 deletions

diff --git a/hugolib/page__position.go b/hugolib/page__position.go
index a087872cc..d977a7052 100644
--- a/hugolib/page__position.go
+++ b/hugolib/page__position.go
@@ -14,6 +14,8 @@
 package hugolib
 
 import (
+	"context"
+
 	"github.com/gohugoio/hugo/lazy"
 	"github.com/gohugoio/hugo/resources/page"
 )
@@ -33,12 +35,12 @@ type nextPrev struct {
 }
 
 func (n *nextPrev) next() page.Page {
-	n.init.Do()
+	n.init.Do(context.Background())
 	return n.nextPage
 }
 
 func (n *nextPrev) prev() page.Page {
-	n.init.Do()
+	n.init.Do(context.Background())
 	return n.prevPage
 }