hugolib: Allow relative URLs in front matter

Before this commit you would have to do this in multilingual setups:

```
---
title: "Custom!"
url: "/jp/custom/foo"
---
```

This commit allows for relative URLs, e.g:

```
---
title: "Custom!"
url: "custom/foo"
---
```

Which is obviously easier and more portable.

The meaning of relative may change to include more in the future (e.g. role based access).

Fixes #5704

resources/page/page_paths.go

@@ -124,6 +124,11 @@ func CreateTargetPaths(d TargetPathDescriptor) (tp TargetPaths) {
 
 	}
 
+	if d.URL != "" && !strings.HasPrefix(d.URL, "/") {
+		// Treat this as a context relative URL
+		d.ForcePrefix = true
+	}
+
 	pagePath := slash
 
 	var (