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

resources/page/pages.go

@@ -132,21 +132,6 @@ func (pages Pages) ProbablyEq(other any) bool {
 	return true
 }
 
-func (ps Pages) removeFirstIfFound(p Page) Pages {
-	ii := -1
-	for i, pp := range ps {
-		if p.Eq(pp) {
-			ii = i
-			break
-		}
-	}
-
-	if ii != -1 {
-		ps = append(ps[:ii], ps[ii+1:]...)
-	}
-	return ps
-}
-
 // PagesFactory somehow creates some Pages.
 // We do a lot of lazy Pages initialization in Hugo, so we need a type.
 type PagesFactory func() Pages