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

tpl/transform/transform_test.go

@@ -14,6 +14,7 @@
 package transform_test
 
 import (
+	"context"
 	"html/template"
 	"strings"
 	"testing"
@@ -185,7 +186,7 @@ func TestMarkdownify(t *testing.T) {
 		{tstNoStringer{}, false},
 	} {
 
-		result, err := ns.Markdownify(test.s)
+		result, err := ns.Markdownify(context.Background(), test.s)
 
 		if bb, ok := test.expect.(bool); ok && !bb {
 			b.Assert(err, qt.Not(qt.IsNil))
@@ -218,7 +219,7 @@ This is some more text.
 And then some.
 `
 
-	result, err := ns.Markdownify(text)
+	result, err := ns.Markdownify(context.Background(), text)
 	b.Assert(err, qt.IsNil)
 	b.Assert(result, qt.Equals, template.HTML(
 		"<p>#First</p>\n<p>This is some <em>bold</em> text.</p>\n<h2 id=\"second\">Second</h2>\n<p>This is some more text.</p>\n<p>And then some.</p>\n"))