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

1 files changed, 16 insertions, 0 deletions

diff --git a/compare/compare.go b/compare/compare.go
index de97690c7..67bb1c125 100644
--- a/compare/compare.go
+++ b/compare/compare.go
@@ -36,3 +36,19 @@ type ProbablyEqer interface {
 type Comparer interface {
 	Compare(other any) int
 }
+
+// Eq returns whether v1 is equal to v2.
+// It will use the Eqer interface if implemented, which
+// defines equals when two value are interchangeable
+// in the Hugo templates.
+func Eq(v1, v2 any) bool {
+	if v1 == nil || v2 == nil {
+		return v1 == v2
+	}
+
+	if eqer, ok := v1.(Eqer); ok {
+		return eqer.Eq(v2)
+	}
+
+	return v1 == v2
+}