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

common/collections/slice.go

@@ -15,6 +15,7 @@ package collections
 
 import (
 	"reflect"
+	"sort"
 )
 
 // Slicer defines a very generic way to create a typed slice. This is used
@@ -74,3 +75,22 @@ func StringSliceToInterfaceSlice(ss []string) []any {
 	return result
 
 }
+
+type SortedStringSlice []string
+
+// Contains returns true if s is in ss.
+func (ss SortedStringSlice) Contains(s string) bool {
+	i := sort.SearchStrings(ss, s)
+	return i < len(ss) && ss[i] == s
+}
+
+// Count returns the number of times s is in ss.
+func (ss SortedStringSlice) Count(s string) int {
+	var count int
+	i := sort.SearchStrings(ss, s)
+	for i < len(ss) && ss[i] == s {
+		count++
+		i++
+	}
+	return count
+}