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, 1 insertions, 1 deletions

diff --git a/markup/converter/converter.go b/markup/converter/converter.go
index c760381f4..7e5b56b07 100644
--- a/markup/converter/converter.go
+++ b/markup/converter/converter.go
@@ -101,7 +101,7 @@ type DocumentInfo interface {
 
 // TableOfContentsProvider provides the content as a ToC structure.
 type TableOfContentsProvider interface {
-	TableOfContents() tableofcontents.Root
+	TableOfContents() *tableofcontents.Fragments
 }
 
 // AnchorNameSanitizer tells how a converter sanitizes anchor names.