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/page__menus.go

@@ -14,6 +14,7 @@
 package hugolib
 
 import (
+	"context"
 	"sync"
 
 	"github.com/gohugoio/hugo/navigation"
@@ -29,13 +30,13 @@ type pageMenus struct {
 }
 
 func (p *pageMenus) HasMenuCurrent(menuID string, me *navigation.MenuEntry) bool {
-	p.p.s.init.menus.Do()
+	p.p.s.init.menus.Do(context.Background())
 	p.init()
 	return p.q.HasMenuCurrent(menuID, me)
 }
 
 func (p *pageMenus) IsMenuCurrent(menuID string, inme *navigation.MenuEntry) bool {
-	p.p.s.init.menus.Do()
+	p.p.s.init.menus.Do(context.Background())
 	p.init()
 	return p.q.IsMenuCurrent(menuID, inme)
 }
@@ -43,7 +44,7 @@ func (p *pageMenus) IsMenuCurrent(menuID string, inme *navigation.MenuEntry) boo
 func (p *pageMenus) Menus() navigation.PageMenus {
 	// There is a reverse dependency here. initMenus will, once, build the
 	// site menus and update any relevant page.
-	p.p.s.init.menus.Do()
+	p.p.s.init.menus.Do(context.Background())
 
 	return p.menus()
 }