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, 3 deletions

diff --git a/hugolib/site_test.go b/hugolib/site_test.go
index 8dac8fc92..a2ee56994 100644
--- a/hugolib/site_test.go
+++ b/hugolib/site_test.go
@@ -14,6 +14,7 @@
 package hugolib
 
 import (
+	"context"
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
@@ -630,7 +631,7 @@ func TestOrderedPages(t *testing.T) {
 		t.Errorf("Pages in unexpected order. First should be '%s', got '%s'", "Three", rbypubdate[0].Title())
 	}
 
-	bylength := s.RegularPages().ByLength()
+	bylength := s.RegularPages().ByLength(context.Background())
 	if bylength[0].Title() != "One" {
 		t.Errorf("Pages in unexpected order. First should be '%s', got '%s'", "One", bylength[0].Title())
 	}
@@ -662,7 +663,7 @@ func TestGroupedPages(t *testing.T) {
 	writeSourcesToSource(t, "content", fs, groupedSources...)
 	s := buildSingleSite(t, deps.DepsCfg{Fs: fs, Cfg: cfg}, BuildCfg{})
 
-	rbysection, err := s.RegularPages().GroupBy("Section", "desc")
+	rbysection, err := s.RegularPages().GroupBy(context.Background(), "Section", "desc")
 	if err != nil {
 		t.Fatalf("Unable to make PageGroup array: %s", err)
 	}
@@ -683,7 +684,7 @@ func TestGroupedPages(t *testing.T) {
 		t.Errorf("PageGroup has unexpected number of pages. Third group should have '%d' pages, got '%d' pages", 2, len(rbysection[2].Pages))
 	}
 
-	bytype, err := s.RegularPages().GroupBy("Type", "asc")
+	bytype, err := s.RegularPages().GroupBy(context.Background(), "Type", "asc")
 	if err != nil {
 		t.Fatalf("Unable to make PageGroup array: %s", err)
 	}