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/embedded_shortcodes_test.go

@@ -14,6 +14,7 @@
 package hugolib
 
 import (
+	"context"
 	"encoding/json"
 	"fmt"
 	"html/template"
@@ -70,7 +71,7 @@ func doTestShortcodeCrossrefs(t *testing.T, relative bool) {
 
 	c.Assert(len(s.RegularPages()), qt.Equals, 1)
 
-	content, err := s.RegularPages()[0].Content()
+	content, err := s.RegularPages()[0].Content(context.Background())
 	c.Assert(err, qt.IsNil)
 	output := cast.ToString(content)