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.go

@@ -15,6 +15,7 @@
 package transform
 
 import (
+	"context"
 	"html"
 	"html/template"
 
@@ -118,13 +119,13 @@ func (ns *Namespace) HTMLUnescape(s any) (string, error) {
 }
 
 // Markdownify renders s from Markdown to HTML.
-func (ns *Namespace) Markdownify(s any) (template.HTML, error) {
+func (ns *Namespace) Markdownify(ctx context.Context, s any) (template.HTML, error) {
 
 	home := ns.deps.Site.Home()
 	if home == nil {
 		panic("home must not be nil")
 	}
-	ss, err := home.RenderString(s)
+	ss, err := home.RenderString(ctx, s)
 	if err != nil {
 		return "", err
 	}